What are transaction fees, and why do they change?

You pay a small fee to miners to include your transaction in a block. Fees go up when lots of people are sending at once and down when it's quiet.

Fees depend on how much space your transaction takes up in a block (measured in virtual bytes), not on how much bitcoin you send. Block space is limited, so it works like an auction. Most wallets suggest a fee in sats per vbyte based on how fast you want it to confirm.