This type of mass does require thorough investigation. You do not mention what the doctors have told you, nor what symptoms you presented with. There are a number of reasons or finding a mass in a kidney.
Two common explanations for such descriptions are harmless cysts or a tumour. Also inflammatory lesions can appear as masses too. Full investigations may include biopsy. Full blood counts, kidney function tests etc will also be required.
You need to be prepared for the possibility that you may have an element of kidney damage, in addition depending upon the exact nature of the problem, you may require surgery or other more rigerous treatment. However, your other kidney can take over a lot of the excretion processes so do not get overly worried about this at the moment.
This article is written from the scientific point of view but covers all the classes of lesion.
I hope this gives you the information you need. I'm sorry that I do not know your history, or what the doctor has told you so far. Please do not sit and worry about the information I have given here. It may not be appropriate for your case but what it does do, is give you all the possible options as they stand at the moment.
If I can help any more, let me know. I'll try and look in on the site tomorrow.