I can explain this!
In English the word "charisma" is an uncountable quality - something that people can possess, such as "charm" or "intelligence." Either you have it or you don't, and you possess it in a certain quantity. However, it is an abstract essence, not a tangible thing, so while a person can "have charisma" or "be charismatic," you cannot "be a charisma."
The difference with the Japanese loanword of charisma (karisuma) is that while it can use the definition I've defined above, it can also be (and is much more often) applied to refer to a person outright, i.e. Freeza is "a charisma." It's such a popular buzzword to use nowadays that there are TV comedians like Ariyoshi who point out the word has basically lost all meaning, because it's used on anyone and anything (particularly magazine models) even if they don't have a particularly notable personality. In that sense, it's similar to the word "celebrity." It doesn't mean that someone necessarily has an arresting and charming personality, but they do attract lots of attention (and it can still certainly be both of those things).
TL;DR: Karisuma = attention grabbing (especially is a celebrity-like way).