An overly zealous anime fan, usually a newer or younger fan to anime who is very loud and excited about anime. These fans are the loud minority of fans that are very upfront and loud about anime in public - unlike the silent majority of anime fans.
Weeboo is also associated with the idea with the idea that this anime fan is excited about anime, but does not know much about anime. This goes back to the generalization that weeboos are newer or younger fans.
These anime fans are the "stereotype" of anime fans which the general public points to as fans of anime. They are the type of anime fans that use Japanese in there everyday conversation or push there love of anime on other people who are outside the anime fandom. These people do not always understand the concept of inside voices.
They are also different from anime fans - or otakus in Western culture - who are zealous about their fandom in private or at appropriate times, such as anime conventions or with other anime fans.
Look at this weeboo running around screaming about Pokemon.
I was just talking to this weeboo who said they love Sailor Moon but couldn't name any of the Outer Senshi.
These weeboos make it look like every anime fan is a hyper screaming five year old.