Main

A main character (or main for short), in the context of Super Smash Bros. is defined as the character that a player uses the most often when they want to win a match.

How to find a main
In order for a player to find their main, they will try out many characters to see whose play style they like the best. People who like to rack up damage with combos may choose characters like Sheik, Diddy Kong, and Zero Suit Samus. People who prefer using projectiles may pick characters like Villager, Toon Link, or Mega Man. People who want to get kills with as few hits as possible and are okay with getting comboed easily will choose heavyweights like Bowser, Charizard, and King Dedede. If someone isn't sure what play style fits them, they will often play several matches with many characters and see what sticks.

After a player chooses a main, they will try to get as good as possible with their character of choice. Ways to do this include going into Training Mode and trying out different situations or playing more matches against human players to learn matchups. For example, if you want to main Villager, you could go ask your friend who plays Mario to play some matches with you to learn the matchup. From this, you can learn things like what Mario's options are against Villager. Since Villager relies heavily on projectiles, a good Mario player will make use of his cape to reflect them back, which can lead to a kill. However, Villager can then pocket the projectiles Mario reflects, which can also lead to a kill.

How many mains should you have?
There are many factors and attributes that each character has, and with so many characters in Smash 4, it can be difficult to choose just one character that suits their tastes. This is why many people have multiple mains. The advantages of this include being able to cover some bad matchups a character may have. For example, one of Mario's biggest weaknesses is his range, which means he has a hard time against characters with swords, like Marth and Cloud. To cover this weakness, you could pick a taller character with more range, or one with a sword. Or both. Basically, just pick Cloud. Cloud is amazing. I find that Toon Link has a really bad matchup against Cloud, so to have a better chance of winning, I pick Cloud too and do a ditto match.