If you're new to the game, you might think to bet the minimum. It makes sense: you don't want to do anything crazy, so why not just tip your toes in the water?
The problem is, a bet that small into a pot that big doesn't make anyone fold. In fact, you might have just gotten everyone more excited and now they have the right pot odds to chase that big hand they weren't really interested in before. You didn't scare anyone out, and you didn't look strong. You made a donk bet.
A donk bet is a small bet made into a big bet that doesn't change the stakes of the hand: instead, it makes everyone think you don't really know what you're doing, and they're all in it to take your chips away from you. You look like a beginner or a lousy player: a donk.
In most cases, the best move is to make a bet proportionate to the size of the pot. If the pot is already really big, you might have to bet quite a lot to do it, but you need to show strength to make other players fold or at least be hesitant.
Now, maybe you're not a beginner; maybe you're at a new game and you've got a big hand in front of you and you want everyone at the table to think you're a donk so they start betting hard into a made hand. In that case, you might just consider making yourself look like a donk, if it means a big payout.