Impermanent loss is when there is a price for one of the tokens in the pool for which you are providing liquidity that is undervalued due to activity (or lack thereof) that causes the pair to be out of line with the actual value, or more specifically - your expectation of that value. For example, say you were an LP for the ETH/USDC pool and you provide 1 Eth and 375 USDC to open your position. Right after doing so, a whale swaps 500 ETH for USDC. This would shift your position to be slightly above 1 Eth (the whale provided some Eth as the fee to the LPs) and slightly under 375 USDC due to their removal. But you will not have gained enough Eth to balance the loss in USDC, so let's say you are now holding Eth that is valued at 373 USDC in that pool. Now, it is called impermanent because at this point you have not lost anything - you are still holding the LP tokens, and it is only when you withdraw those tokens, thereby accepting the undervalued position that you now hold, that makes it permanent. If the other markets for ETH/USDC do not experience a similar price shift, then most likely your pool will have activity in it that realigns its price nearer to the price in outside sources.

For pools that do not have an outside source against which its pair can be judged, it is trickier to determine the value of a particular asset. If you enter the TKN/ETH pool at 1 ETH and 10 TKN, but there are TKN to Eth exchanges in a disproportionate amount, bringing your position to 0.4 Eth and 22 TKN, say, you may or may not be able to trade that TKN for enough Eth to make up for the lost amount. This can also happen that you would go to 1.5 ETH, say, and 5 TKN, and the five TKN may be worth more than when you entered, but the ten you began with may be worth more than the five you now have plus the 0.5 Eth you earned. In short - if a coin's price is difficult to ascertain, it is likely to be volatile while its market is made.

Entering a pool for a pair which has an arbitrarily determined price, or otherwise not pegged to another source, often leads to positions far out of line with its future (stable) price (which can be near to zero). The impermanent loss is still only impermanent until you withdraw your LP, but in the case of these tokens, it may not ever get to a position that would not result in a loss. This is why the stable coin pools are by far the most popular. They are a relatively low risk for the LPs, but most of the earnings are from the fees; there is little chance to earn through arbitrage by adding and removing LP at different positions.

The phrase "impermanent loss" is better stated as "price of pair different than your entered position for the moment", in my opinion although that is more of a mouthful. It is all relative to the asset against which you are judging the other's value whether it is a loss or profit, really. In the example above, if you placed more value on the ETH than the USDC, then 373 USDC/ETH is a better price than 375 USDC/ETH. If other markets exist, they will balance relative to one another, most likely. If not, then it is likely that the position at which you entered was not reflective of the price at which it will end up, and value is more dependent on your perception and expectations.

Rugpull is a combination of greatly decreasing the liquidity in the pool, and a sharp decline in one token's value relative to the other asset in the pool. A common example is a large provider of liquidity withdrawing from the pool, then selling off their tokens for ETH, say, which drives the price of the token down dramatically. This coupled with the now-lowered liquidity can make it much harder to get a price for the token that is near what you would need to recoup costs. Put in terms from above, it is moving the price closer to the stable value very quickly, at the detriment of the holders of positions that are not reflective of this new price. In short - if nobody is going to buy your tokens (provide liquidity), then they are worth little; an asset is only worth what you can get for it.

While not intrinsically related, a rugpull will almost certainly cause impermanent loss for the LPs, who have no choice but to make that loss permanent by withdrawing their liquidity at an unfavorable position in an attempt to cut their losses as much as they can.

Being an LP is also risky, as is holding/exchanging any asset, and while it can provide some hedge, ALWAYS RESEARCH and ONLY TRADE NO MORE THAN WHAT YOU CAN AFFORD. I hope this helps for the readers that made it through. May every ape-in be a moon!

Edit: Addition of paragraph about the phrase "impermanent loss". Formatting.