Hi and welcome to garden.org. If it is a sprout growing below the graft on an existing lemon tree, then it will probably be a sour orange, which is the root stock that most citrus are grafted onto. IF your big lemon is not grafted, (eg. you grew it from a seed) then it might be also a lemon.
It will take at least 6 years, maybe longer before you will see fruit from that and in the meantime it will steal nutrients and water from its mama. I would nip it off, if it were mine.
Btw, if you would please complete your personal profile with your location (city, state, country) it helps people to answer your questions. Click on the little person icon in the blue sidebar to the left.
