A shower diverter valve is the part that sends water from the tub spout up to the showerhead in a tub and shower combination. When the diverter is off, water leaves through the spout to fill the tub. Engaging it blocks the spout, so the water has nowhere to go but up the pipe to the shower.
There are three common types. A tee or lift-gate diverter sits in the tub spout: you pull up a knob and a small gate seals the opening. A two-valve or three-valve setup puts the diverter on the wall as its own handle, the second handle or the middle of three. In many single-handle valves, the diverter is a separate knob on the trim or is built into the cartridge.
A failing diverter usually shows three signs. Water keeps running from the tub spout while the shower is on, because a worn gasket or gate no longer seals it. Shower pressure feels weak, since part of the flow escapes below. The knob or handle gets hard to turn or drops back down mid-shower. Mineral buildup and dried-out rubber seals cause most of these failures.
The fix depends on the type. A worn spout diverter is usually solved by replacing the whole spout, because the gate is built into it. A diverter inside the wall valve means removing the handle and trim, and the new part has to match the valve brand and model, which makes it a bathroom plumbing repair rather than a quick swap.
One honest limit: weak shower pressure is not always the diverter. A clogged showerhead can cause the same symptom, as covered in why shower pressure suddenly drops, and if every fixture is weak, the cause is low water pressure in the whole house, not the shower.