The frame is the solid part of the bicycle that interconnects the various components. Frames come in various materials, including steel, aluminum, titanium, and carbon fiber. The most popular types of frame are the double diamond and step through frames.
Frames can become cracked or bent and require repair or replacement.
Steel frames can be readily straightened, other materials can't generally be straightened. Tools exist for the straightening of frames, or many can be improvised.
Steel frames can be welded, other materials can't generally be welded.
The paint on a frame can become chipped, etc. and need to be repaired.
Sometimes components can become seized in the frame and require removal.