What does XV mean in Rome?

I think you mean in Latin (the old roman language - mother of Italian, Spanish, Portuguese, etc, etc, etc.)

It's a numerical abbreviation... Romans didn't use the Arabic numeric system (or maybe it's from India - but nobody knows it for sure).

I = 1, II = 2, III = 3, IV = 4, V = 5, VI = 6, etc.

X = 10, L = 50, C = 100, D = 500, M = 1000, etc.

XV = 10 + 5 = 15 (your question)

It's quite easy... don't forget that IV (one less than V) stands for 4, IX (one less than X) stands for 9, etc. Now you're a professional in latin-number-spelling ;-)