It’s a sexual variation in which the person gets sexually aroused by smelling, feeling and tasting urine or by urinating on someone or being urinated on. This activity is often described by euphemisms, like ‘watersports’ or ‘golden showers’.
With some women orgasm is more intense if they have a full bladder and they urinate while they come.
For some being urinated on is a turn on, others like to urinate on their lover’s body. This can be a purely physical pleasure, enjoying the warmth and smell of it. But it can also have to do with intimacy, closeness and trust or it can be part of a sadomasochistic game, having to do with dominance and submission.
Some like to pee in their pants (‘urolagnia’) or watch someone else do this. This can be part of a game in which one pretends to be an infant, possibly wearing diapers, while the other person takes the role of a parent.
Some like to drink urine (‘uriposia’). By the way, the smell and taste of urine changes with what people eat and drink. If you have ever eaten asparagus you’ll have noticed this.

Safety

I fantasize about peeing on my lover and would like to try it. But how do I ask?