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Damage spread refers to the fact that a weapon has a spread, or range, of damage that it can deal on any given hit. By default, weapons generally have a +/-10% damage spread.

So if a weapon has 100 base damage, it might do as little as 90 (10% less than base) or as much as 110 damage (10% more than base) on hit. This amount of damage dealt is simply a random value from within the "spread".

Originally, the spread was +/-25%, but Valve eventually reduced it to only +/-10%.

Note that this isn't the only thing that affects weapon damage, there is also a damage falloff based on range for most weapons (they do less damage at greater distance).

Some servers disable damage spread because they think it makes the game more "skill-based" and more fair, since damage dealt is more consistent. This is similar to the argument used for disabling critical hits.

"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.