What does it mean to “top deck”

"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.

L-Canceling via Smash Bros Wiki. L-canceling does not work in Brawl, although auto-cancel does.

To L-cancel, press L, R, or Z (only Z and R work in SSB) within 6 frames of landing on the ground while the character is still performing an aerial attack. If performed correctly, the character's recovery animation will be played at twice its normal speed in SSBM, effectively cutting recovery time in half. If the number of frames of the lag divided by two ends with a decimal (i.e. 15/2 = 7.5), it will be rounded downwards. In SSB, however, the recovery animation is erased completely, allowing one to move immediately after any aerial attack. L-canceling can only be used with aerial attacks involving the A button; it is impossible to L-cancel a special move. It is also impossible to L-cancel Mr. Game & Watch's neutral aerial and up aerial attack.