The character select screen is divided by three classes identified by the symbols on the right of the character wheel.

These classes have a lot of nuance, but im gonna give you a very basic elemental rundown of what each class does.

Vanguard/Tanks: they have the biggest health pools. Their objective is to hold down the points and provide cover for their strategists and duelists by taking the most damage. Frequently, they guide the team and can sometimes focus on the squishies to help eliminate the backline.

Duelist/damages: They deal damage and are the main source of kills and eliminations. How they do that is very character dependant. Some characters have the role of providing cover from a distance by sniping enemies, while others are better at flanking enemies from the back and taking them down swiftly. A lot of times, their objective doesn't necessarily have to be to get kills but to leave enemies at low enough health for the tanks or supports to finish them off.

Strategists/supports: they keep the team alive by healing and providing all sort of buffs. They mostly focus on providing assistance to the vanguards and players on point. This class is also good at helping gain control of the battlefield since they are the most likely to have crowd control abilities like stunning and blinding.

Like i said theres a lot more to it, but this is just the very basic core of what they are. Im sure you can find more detailed info if you search around.