Question

what are galvanic cell? Give the function of salt bridge?

Answer 1

A galvanic cell is an electrochemical cell that converts chemical energy to electrical energy. The reaction is spontaneous unlike that of electrolytic cells. 
And for your second question:

Answer 2

Explanation:

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$\longrightarrow$ A commonly used form of salt bridge consists of a glass U- tube containing semi- solid paste of either KCl, $KNO_3$ or $NH_4Cl$ in gelatin or agar-agar jelly.

$\longrightarrow$ The electrolytes that are often used in salt bridge are called inert electrolytes which are supposed:

not to interact chemically weather of the solutions present in a anodic or cathodic Chambers.

not to interfere with overall cell reaction.

only those electrolytes can be used in a Salt Bridge in which mobility of Ions is almost the same.

Example, KCl, $K_2SO_4$, etc.

$\longrightarrow$ A salt bridge carries out two important functions:

It allows only flow of Ions through it. Thus, the circuit is completed.

It also maintains the electrical neutrality.