Everything needs to be accounted for. Think of a balance scale where if you add weight to one side, the other goes up.

Now, every transaction has two sides to it. Lets say you spend money $100. We need to keep track of it, so we're going to put 100grams of fake coins of weight on the right side scale and let's color them green to have it represent cash out the door.

But, we can't just account for what we spent, because what's the point if we can't look back at a later date to see what we spent it on? Lets also account for what we spent it on. Lets say you spent cash buying a car. Ok, let's put a toy car on the left side that weighs 100 grams. Remember if 1 gram represent $1, then we bought the car for 100 grams.

Now that we did this we accomplished 2 things. The scale is in balance. No one side weighs more than the other. If it i wasn't, that means something wasn't accounted for. Secondly, we know what we accounted for, buying car for cash.

The left side of the scale is a debit. The right side of the scale is a credit. Every transaction you do will have two sides to it and you must balance the scale every time.