AFK is the term to call away from keyboard. It is the situation where you are not on the screen but people tend to see you are on the screen with the help of something like bot. Players usually use the AFK bot to pretend that they are on the screen while they have something to do like they have to go to bathroom, and so on.
AFK is not allowed and it breaks the rule. If the other players know that you are AFK and they report you, your account will likely to be banned. If you care about the safety of your account, it is better for you to not to AFK. So, if you have a plan to level up while you are AFK, please cancel your plan and just do the normal way.
If you want to level up your Fortnite account, all that should b done is to earn XP. The main way to get XP is from playing matches and more being rewarded based on the performance in matches. Apparently, your performance indicates the amount of XP you will earn. The sources of Fornite XP include first kill (50 XP), additional kills (20 XP, capped at 100 XP, so only six kills total reward XP), survival (17 XP each minute after the first circle shows up, capped at 14 minutes or 238 XP), top 20 placement (25 XP), top 10 placement (100 XP), and Victory Royale or 1st place (300 XP).
There are certain challenges to complete along with medals that can be used when you are not focused on certain challenges. If you want to get extra XP and level up fast, here are some things that you can do.
The first one is to complete the daily challenges. Every day, you will get a new challenge, and you can have up to a maximum of three at any one time. Each of them is able to range from getting catching items while fishing to looking for chests in a specific location and many more, and completing each one rewards you with a certain amount of XP each time. Even if cannot complete the challenge that day, you need to make sure you at least log on to collect it or check out which ones you have been dealt that day. Do not forget that you are also able to re-roll one challenge each day if you are not fond of what it is offering. You are able to just roll it if you need to place top 50 in solo three times but you only play with your friends. If there is a great chance you will be granted an objective that is more suitable to your style of play.
Some other ones are marking those challenges and earn rewards with the Battle Pass, playing with your friends and use Party Assist to help complete challenges quicker, and earn medals for smaller XP boosts. Feel free to try all the methods.