Okay kiddo, so back in the 1960s, there were a group of rock bands from Britain that became super popular in the United States. They were called the "British Invasion."
But there was also a group of comic book artists from Britain who became really popular in the United States around the same time. They created superhero comics that were different from the ones Americans were used to.
These British comics were more realistic, with characters who had their own problems and flaws. They were also drawn with more attention to detail, which made them stand out.
Some of the most popular British comic book artists were people like Alan Moore, Dave Gibbons, and Neil Gaiman. They created comics like "Watchmen," "V for Vendetta," and "Sandman," which are still really popular today.
So, when people talk about the "British Invasion" in comics, they're referring to this group of British artists who made a big impact on American comic book culture.