mochi shop; mochi seller
Refers to a store that sells mochi (rice cakes) or the person who runs it. The context usually makes the meaning clear.
I bought a kagami mochi at a mochi shop before New Year's.
The owner of that mochi shop is very kind.
Compound of 餅 (mochi, 'rice cake') and 屋 (ya, 'shop' or 'seller').