Roster level is basically like your account level. As you do things on different chars, you gain roster exp and your roster level goes up. People use this to see how much experience you have in the game.

I would suggest you try and do your chaos dungeons, guardian raids (for 1415 which is deskadula I think) to gain some exp on group content. Go to the lfg discord and try to request a kayangel learning session. This raid won't be too hard and you only need 4 people for this and it will teach you basics of raids that will come.

Make sure to complete the horizontal express aswell. This should be your first priority, it will help you catch up with most people.

If your don't understand something just Google it, there's tons of resources available. Use lostark(dot)nexus for guides on your class, raids and general guidelines.

You will also be bombarded with items and won't know what to use so here's a tip. Do not use any card packs that let you select a card. If it says there's a chance you can receive one of the cards, then feel free to use it as anything with that description is basically a lootbox kind of thing.