Often in literature and tutorials the concept of a "Frame" is used. In TF2 a frame requires both a parent and a child name.
So when someone refers to "frame" are they referring to the name of the parent or the name of the child?
I think the answer is best presented with a simple example. A robotic arm is a chain of frames (within transforms) from the base to the end effector. Each frame is defined relative to its parent. For example:
[ Base frame (parent) -> Link_1 frame (child) ] First transform
[ Link_1 frame (parent) -> Link-2 frame (child) ] Next Transform
[ Link_2 frame (parent) -> End_effector frame (child) ] Last transform
When referring to a frame in this system, we usually mean the child frame. For example, when referring to the end_effector frame, we are talking about the frame which is the child frame in the transform from the last link of the robot to the end effector.
The name frame is short for coordinate frame. In any system there are many relevant coordinate frames.
A transform is a representation of what it takes to move data represented in one frame to the other. (Note that this is the inverse of what it takes to move a frame to the other frame. )
So transforms represent the relationship between two frames, using a tree such that you have parent and child to be able to traverse the tree between any two frames and compute the net transform. So transforms have parents and child ends but frames do not.
With that net transform you can transform data from a representation in the source frame to a representation in the target frame.