Recalling is when you teleport back to your base - kinda like using a hearthstone in World of Warcraft. It takes a few seconds to complete and you're vulnerable while doing so which means you need to be careful where you choose to use it. The time factor is important because the risk of getting hurt balances the reward of saving time to go back home - it's not a get-out-of-bad-fights-free card.
Thing is, this Recall bug is letting a long list of champions teleport near-instantly which obviously makes it incredibly overpowered.
The bug involves rapidly using and cancelling channelled abilities and auto-attacks in conjunction with the Recall so the problem is widespread but the exact sequence and complexity varies from champion to champion.
Originally it was thought to only affect Riven so Riot had temporarily disabled her in the game. But as more and more champions were noticed to have some version of the problem (23 are listed on the subreddit megathread) Riot decided to simply switch off ranked mode until a fix can be found.
There have been occasional mentions of an instant recall bug at various points over the last few years but patch 5.13 which was recently deployed seems to have brought the problem into the mainstream (god, I remember when instant recall was so underground etc etc).
Riot also has this notification on the service status update page:
"We're currently working to fix a bug with the Recall spell. Unfortunately, this particular issue can't be mitigated by disabling a specific champion or item so we've temporarily disabled Ranked while we develop a solution. We will update you all with our progress in 12 hours."
12 hours will be about 2pm in the UK. In the meantime you can still play normal queue but it's an "at your own risk" deal.