When something is made with two parts that must be fitted together, often one part has a hole in it, and the other part has a little piece sticking out, often rectangular in shape. This latter part can be called a 'tongue'.
Think of a door. The part of the lock that slides in and out is 'the tongue'. You open and close the door by turning the handle, which slides the tongue
I must admit, I don't find the expression "tongue of frame" at all familiar. "The tongue of the frame" seems more likely; but in that case, the "frame" is redundant, as every "tongue" must be attached to something.