Expose function in ruby on rails

I am looking at a rails app and at the top of every controller there is a block of code that looks something like this

expose(:var) {Model.find params[:var_id]}

I understand what is inside the block just fine but...

I cannot find any documentation on what the expose function does where it comes from or anything I have tried searching the project and using the searchable rails docs.

I would love to know what it does, can someone please tell me or point me to the docs.