The role of a salt bridge in a galvanic cell is to:

The external wire carries the electrons; the salt bridge carries ions between the two half-cell solutions. Without it, accumulation of positive ions around the anode and negative ions around the cathode would polarise the cell and the current would stop. The salt bridge neutralises this build-up by supplying inert ions (typically $\mathrm{K^+}$ and $\mathrm{NO_3^-}$) and completes the electrical circuit.