"Age disparity in sexual relationships" means that there's a big age difference between two people who are having a romantic and sexual relationship.
For example, if a really old man is dating a very young woman (or vice versa) and they're having sex, that's an example of age disparity.
Some people think that having a big age difference in a sexual relationship isn't healthy because the younger person might not have enough life experience to make good decisions and might not be able to stand up for themselves if they feel uncomfortable.
Other people think that age doesn't really matter as long as everyone is happy and consenting.
It's important to remember that if you're ever in a situation where you're uncomfortable or feel pressured by someone who is significantly older than you, it's okay to say no and talk to a trusted adult about it.