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.

but don't nip, instead pull, so you get the entire sucker.

I don't think that is a tree next to it, is it?

Eek, you're right, Pork. It's a PVC tree. I didn't embiggen the picture.

So, (backpedaling furiously ) if that is a citrus that started from a seed, you still won't know what you've got for years, until it blooms and bears fruit. Years and years. If you want to grow a tree for that long and take a chance that it's something worthwhile . . that's up to you but I wouldn't.

If you really want a lemon tree, go to a nursery and buy a grafted one that is for sure going to give you nice fruit, probably within 2 or 3 years.