(90+7)
Coming from football, it indicates the extra-time given at the end of a match. As a meme it's used to express something which happens unexpectedly or in the last minute. The number after the "+" can be any that realistically gets added as extra-time (usually not more than 10)
Mom:*tries to check my search history*
Me: destroys the phone (90+7)
Me: destroys the phone (90+7)