Very very new poker player here, why are donk bets bad?

Hi all. I'm super new to poker, and I pretty much only play online with monopoly money or 1c 2c games with friends. Not really playing for money, I just find the game fun.

One of the (many) things I don't understand currently is why a donk bet is a bad idea. If I've got top pair on the flop, say 10 or something like that, why would I check and let the other player check again, effectively letting them see the next card for free.

The way I see it, if I have a good hand, I want to bet, so that there's more of a reward for winning the pot. If I just check and they check back that's just leaving potential winnings on the table.

And if my position is kinda tenuous, like with the top pair 10s example, wouldn't I want to bet big to encourage the other player to fold, instead of just letting more cards come down that could weaken my position?

Sorry if this is a stupid question, I'm just very new.