New owner here, what is this thing?

I traded in my 2017 F150 EB for a 2025 F150 Lariat PB and I’m still figuring everything out. I found something and I don't know what it is. It’s circled in blue in the picture and is on the driver’s side rear end. I looked through as many threads as I could and didn’t see anything about it. Any help would be appreciated!