The term "string bet" refers to a player putting chips into the pot in multiple moves instead of just one motion.
For example, if you are playing a $2/$5cash game and want to bet $100 but firstly slide $50 into the pot with the intention to add $50 right after that, it is string betting. You failed to put all the chips you intended in one motion, so only the first part of the bet will count if you did not announce it verbally.
That’s why you should always say how much you are betting in advance, and then you can add the chips in the pot whichever way you want since everyone knows what the bet is. This will save you from possible confusion or even some trouble at the tables.
A string bet is considered an unethical move and is not a legal bet, meaning that only a part you make with the first move will be counted towards the game.