Itenimum1
The uid tells what other object to connect to. I could have used an object type instead, but using a uid instead makes stuff like connecting to other instances simpler. As an example if you want to add a joint to Sprite1 to connect to Sprite2 you would use Sprite2.uid.
For the xy's they define where on the objects the joint is connected.
"rect" and "polar" are offsets from the object, but it's also relative to the object's angle. So for example if you used "rect" with x=32, y=0 and the object had a angle of 90 then the object would be connected 32 pixels below the object's center.
"imagepoint" should be self explanatory.
"layout" is a layout position, which in turn is used to calculate an offset. It's probably simpler to wrap one's head around than "rect" and "polar", since it's not relative to angle.
Depending on the joint there usually are two anchor points, one for each object. The only exception would be "add anchor joint at XY", but since ever field has a description so it should be understandable.