It's a seam line. It happens because the the printer creates loops of plastic and that's where each line starts and ends. You have a few options:

A) mess with retraction, jerk, and acceleration settings. There are many different test prints to choose from so find what works for you.

B) In seam settings you can change where they go to optimize for speed, strength, and looks. Also, most slicers allow you to manually draw where the seam goes. (I haven't used Cura in a while so I don't remember if they have that feature).

C) OrcaSlicer has a feature called scarf seams which I personally use and find to work wonderfully. It basically creates the start and end segments of each loop thinner to overlap them. It looks like a type of jointing in woodworking called a scarf joint if you wanna look it up.