"Metroidvania" is a term used to describe a sort of exploratory 2D platforming game. It comes from the original games of the Metroid series and some of the middle-era of Castlevania games (most notably "Symphony of the Night", which is what primarily gave birth to the term).
The most defining feature is usually the world. Rather than have set stages or a world map, the entire game takes place inside one giant map, which you must traverse about. Not just any map, but typically a very complex map filled with all kinds of secrets. The map is typically divided into sectors, more in an aesthetic fashion to denote that the denizens and dangers will be different. Sometimes you have teleports to assist you in travel, but other times you just have to remember where to go in the map.
There's various elements at play in the exploration. There's typically tidbits hidden in spots, or puzzles that have to be returned to when you find appropriate equipment. The exploration is also non-linear - your limits of progression through the map are by various tools or powerups that you need to acquire, but the order in which you must acquire some of these tools is not set. And in many choices, there are many paths to get those tools, or even get past obstacles without those tools.
A very popular element of these games, especially in the Metroid franchise, is the concept of "sequence breaking". Using special tricks or otherwise mastering the utility of your basic abilities, you can sometimes access items and areas far earlier than the game expects you to.
As far as I know, Metroidvania games have always been 2D platforming games. Aside from the elements of exploration, the actual gameplay of Metroidvania games vary widely.
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A reboot is a way to reuse the concept behind a game/movie/etc. (backstory, gameplay mechanics, some characters/locations/plotlines) but otherwise rewrite the entire plot and characters. It's used to generate interest based on the franchise it's in, or to reuse a concept that was interesting but might be too old to make a sequel for or originally too poorly executed. It ignores and can totally change accepted canon (ie the characters and plot already established by previous ganes) if they're irrelevant to the new story (which is the difference vs. a remake) and often gives rise to a form of multiverse theory among some gamers.
A good example outside of gaming is the new Robocop movie. It's based on the same premise of a man who was seriously injured, stuck in a mostly robot body, and made to be a cop, but many of the other characters and much of the story was written from scratch.