Definition of string bet
A string bet in poker is an illegal bet. It is the act of betting in several steps. Imagine a virtual line that you cross when you make your bets. If you want to bet €600 for example, you can't take a €100 chip, bet it, wait to see your opponent's face, add another €100 chip, assess his reaction again. When you bet, you have to do it in one step. You take the chips you want to bet and move them to the other side of the line in one go.
Tips to avoid the string bets
It's possible to be a bit in the moon and not take the right denominations and not make the right bet. For example, you want to bet €200 by betting 2 chips of €100, but instead you bet 2 chips of €50 for €100. To protect yourself and to play in a transparent manner, always announce the bet you want to make. By announcing aloud "I bet $200", the dealer will probably let you add your extra $100 if you only wager your two $50 chips instead of two $100 chips.