A while ago Gmail introduced a feature where “important” emails could be automatically detected and then given prominence with a yellow marker.
It’s a good feature for those who need it, but you may not like the look of it. To turn it off, you need to head to the Gmail settings screen by clicking on the gears icon then clicking “settings”.
Then head to the “inbox” screen, and then click on “no markers” in the “importance markers” section. Whenever you want to turn them off, you can just flick it back.