Step 1: Charge imbalance that develops during cell operation.
At the anode, oxidation releases cations into solution, making that compartment progressively positive. At the cathode, cations are deposited, leaving excess anions. This growing charge separation would stop the cell.
Step 2: How a salt bridge restores electrical neutrality.
A salt bridge (a gel saturated with $KCl$ or $KNO_3$) allows ions to migrate between half-cells: anions flow toward the anode compartment and cations toward the cathode compartment, neutralizing the charge buildup in both.
Step 3: Other essential functions.
The salt bridge also completes the internal electrical circuit, prevents direct mixing of the two electrolyte solutions (which would cause spontaneous direct chemical reaction), and minimizes the liquid junction potential.