This English idiom actually makes no sense if you try to take it literally. However, figuratively speaking it means to contribute (give) to something or someone or to join in.
So if your dad tells the family that he wants everyone to pitch in this weekend and help clear the backyard, it means he wants everyone to join in on the efforts to clear the yard and get things done quicker.
“What are you going to buy Sally for her birthday?”
“I don’t know I don’t have much money.”
“Maybe we can all pitch in and buy her something great.”
The above conversation suggests that every one of Sally’s friends should contribute a little bit of money so they can afford to buy her a bigger and better present together.