Okay kiddo, have you ever watched a movie where everything looks old and people are wearing old fashion clothes? That's like the opposite of anachronistic.
Anachronistic is when something doesn't fit in with the time it's supposed to be in. Let's say you were watching a movie about knights and then suddenly a character pulled out an iPhone to make a call, that would be anachronistic because smartphones didn't exist during the time of knights.
Or let's say you went to a dance party and the DJ played music from the 1700s, that would be anachronistic because people don't normally dance to that kind of music nowadays.
So anachronistic means something doesn't belong in the time period it's supposed to be in. It's like if you wore a Halloween costume in March, it just doesn't make sense!