What is a closure when profiling?

Question

Profiling

Hello! With the help of Xhprof and xhgui I look at what resources are going in the request and I am interested in the following points: Self Wall Time
column : Doctrine\MongoDB\Cursor::Doctrine\MongoDB\{closure} and execution time, 6 sec (for example) Memory Hogs column : that same process and closure. 220 mb takes. I understand this is doing something to Mongo, but what is meant by {closure} ?