Boquerones en vinagre is a Spanish dish made with small white fish called anchovies that are marinated in vinegar.
So, imagine you have a tiny little fish, like a toy fish you might play with in the bathtub. Someone takes that fish and puts it in a special liquid called vinegar, which is kind of like really sour juice. They leave the fish in the juice for a while, which makes it taste kind of tangy and a little bit sour.
After the fish has been soaking in the vinegar for a while (maybe a day or two), someone takes it out and puts it on a plate. They might sprinkle some herbs or spices on top for flavor, and then they serve it to you as a snack or a small appetizer.
So basically, boquerones en vinagre is just a fancy way of saying "tiny fish soaked in sour juice and spices." It might sound a little weird, but lots of people really like it!