can anyone explain to me in simple terms what happened to Lewis and Mercedes this reason?
If you mean "season," then yes, I can. His car is not as fast as the others. There was a massive rule rejiggering (massive by F1 standards) and Merc's answer wasn't up to the task. This is no unusual event and many times teams have started with subpar cars only to improve them as the season progressed.
What was different here was that the new formula was aimed directly at Merc and specifically Lewis getting his 8th WDC. Those ghosts are still clutching their pearls because Merc is getting faster and Lewis is getting mad. A Black driver, an unapologetically Black driver with the WDC record? Oh dear, oh dear.
**edit - as an example, Ferrari barely podiumed last season yet this year they're as racey as fuck. Of course, they're known for cheating so we'll see how that works out. And Lewis should have won the WDC last year if the race director had any balls and didn't cave to horner's whining. Also, I'm sure the big boys whispered in his ear about don't let Lewis win if at all possible.