a person who does not have a fixed role in a team, and is versatile enough to switch roles at a moment's notice and still fulfil their new role with sufficient proficiency. they are also known as generalists, and can help balance a team to reduce their weaknesses or vulnerability, particularly if an enemy team is exploiting said weakness.
contrast this with a 'normal' person, who will lock into and serve a static role throughout the team's existence.
it is commonly used in sports, and gained popularity in the realm of eSports, where class-based games provide an explicit benefit for designating flex players to allow their team to adapt. two concepts of flex players arise:
1) in games where classes* are locked in at the start of the match, such as MOBAs (DOTA, League), flex players often choose classes that are known as 'flex picks' in their communities - classes that are inherently versatile enough to be able to switch roles on their own.
2) in games where classes* can be changed at the start of a new round or mid-match, such as FPSes (Rainbow Six: Siege & Valorant or Overwatch & Team Fortress 2, respectively), flex players can either choose 'flex picks' (explained above), or will change classes throughout the match as the needs of their team evolve.
*see also (game-specific terms): heroes, champions, operators, agents, heroes (yes, again), mercenaries
#1)
jungler: 'im getting focused down, my lane needs help'
flex player: 'alright, ill flex over to that lane right now'
#2:
teammate: 'our map control is too weak and the enemy team is exploiting that by rushing us down each round.'
flex player: 'got it, let me switch to an area denial class next round.'
A guy who plays draft normal games and gains LP. Totally irrelevant.
-Hey, i'm, challenger.
-Ok, but you're a flex player.