In live poker, a “string bet” refers to when a player doesn’t make their bet in one forward motion and also doesn’t verbally announce the size of their bet.
Casinos and live card rooms typically have a house rule against string bets to prevent players from trying to gauge the reaction of his opponent during the motion of moving the chips and adjusting the amount they bet based upon their opponent’s reaction.
Players must declare the amount they want to bet, or make the bet in a single motion across the bet line. Novice poker players are frequently string betting without knowing they’re doing it. For example, “I’ll call your 1,000, and raise you 10,000.”
Why is this not allowed? Well, both those bets are made up of two separate movements.