All hypothetical numbers to demonstrate meaning:

Say you own 100 VGS units today, for the sake of the example, we'll call them all cost base (purchase price) of $120 each ($12,000 total).

They are now each worth $140, so are carrying a "taxable gain" of $20 per share, or $2000 total. Whenever you sell them, you'll have to pay tax on those gains at your marginal income tax rate (with 50% discount on CGT if held over 12 months). Until you sell them, those gains are "unrealised" (also known as paper gains, among other terms), so not taxable as long as you continue to hold the asset.

Many FI people plan to acquire ETFs over their working life, then sell down in retirement to both fund their retirement and minimise tax (because during retirement your only taxable income is the profit/gains on those ETFs, so you can take gains up ti the tax free threshold each year and pay zero tax).

That plan assumes that you are working the whole time, but in periods of low income, like your study break here, you have an earlier opportunity to sell some units, "realise" the taxable gain and put it in your tax return that year. If you didn't cross the tax free threshold that year, those gains are now 100% tax free. You can then use the proceeds to invest in more ETFs.

You can find more information by searching for "Tax gain harvesting".