What does"here you/we go" mean?

[hamit1987]

When watching English movies ,I always come across this expression "here you/we go". I just guess its meaning every time , but don't know if I understand it correctly or not. So , could you please explain its meaning to me with the following context ? Thank you very much!
Context1: Lizzie is looking for Kate. Now she finds her and says:"Here you go Kate, this is your invitation card"
Context2:John,a teacher, checks students' attendance first,then says:"Here we go class, let's start our new lesson!"
Context3:Jim is a driver. He picks up his friends from a restaurant and says:"Here we go guys!" Then he drives his car. After some time , they arrive their destination and the driver says again:"Here we go!" Others get off the car.

[Ouisch]

In your first two examples, "here we/you go" are simply introductory phrases. In AmE, "here you go" is commonly used when presenting something, either physically or metaphorically.

"Hey, everyone else got a cookie except me. Where's my cookie?"
(Teacher hands him a cookie.) "Here you go."

***

(On the first day of class):
Teacher: "This semester we will learn all about Greek mythology. Are you ready? Here we go!"

***

"Here we go" can also be used literally, as in we are about to commence movement of some sort. Such as in your third example when everyone piles into a car and the driver announces "Here we go!" as he accelerates. Once they've arrived at their destination, the metaphorical definition applies - "We've arrived at our destination, we don't know what awaits us, but here we go! Let's see what happens."