New neighbors across the street are saying part of our yard is theirs?

So we have lived here for four years and there have never been any issues with neighbors however an older neighbor once mentioned a 10 foot or so section of our yard actually belonged to her even though her house is across the street. No big deal and our realtor never told us anything like that when we bought the place but she died and our new neighbors moved in and are using our grass lawn as their personal parking space . What do we do? Is there anything we can do? What if they do own it? This is our first house and we bought it knowing nothing about it really (we don’t have family to help us with stuff like this) . It makes our house and yard look like an eyesore. I didn’t know someone else could even own part of my lawn .