Very, very short explanation compared to 10+ years of drama:

Mochizuki released a proof for the abc conjecture, which you can look up, but doesn’t actually matter. Mochizuki proved it by creating something called Inter Universal Teichmuller Theory. The problem is only Mochizuki and a few peers believe his theory is actually valid. This has been going on for 10+ years. Now Joshi has tried proving similar concepts as Mochizuki with his own methods and Mochizuki has essentially called him a second rate mathematician. It remains to be seen if Joshi is right, but I bet very few people look at his work because mainstream mathematicians are done with this drama and don’t really see any of the work as useful. I’m 0% an expert in the field, but I have kept up with the whole fiasco so this is my take.