What even is Flex?

https://gyazo.com/2c6c98a08786987db7a8dbafa4baeb00 https://gyazo.com/a0002507112a9c99da458281b3636730

What exactly justifies the MMR difference? Just curious, not really complaining. Games like that just aren't fun from teams tilting before the game even starts. Is it as simple as say Flex Threes doesn't look at any other Leagues you played and the SoloQ challenger had pretty much the same threes MMR as us?