It is a conjunction used to introduce a clause that describes the time or circumstance in which something happens.
Here are some examples:
* Jab woh ghar aaya, tab usne mujhe dekha. (When he came home, he saw me.)
* Jab tumhari umar 18 saal hogi, tab tumhe vote de sakte ho. (When you turn 18, you will be able to vote.)
* Jab barish hogi, tab hum ghar mein reh sakte hain. (When it rains, we can stay inside.)