Select then and delete them.
A more or additional 3d view and the full screen with all the relevant panels may help understand what they are. Is there a mirror mod on? Are two axis in use? All the actual info that might tell viewers something.
To expand on NP5’s point:
- These could be the result of extruding an edge loop outwards, creating “faces with no thickness”
- These could be rogue edges that don’t even form faces at all
- These could also be an “accordion fold,” with the bottom and top faces on top of each other and z-fighting.
Given that you’re doing the pillars in Grant’s beginner course, it’s most likely the last one (I had this problem myself), but without additional perspectives in the screenshots, we can’t be sure.
Deleting will work fine in all 3 cases, but it will leave holes in the mesh in the 3rd case. Filling holes isn’t difficult, but it’s not covered in this part of the course (not sure if Grant covers it at all in this course actually…). To avoid making holes, you can try to dissolve this topology first; if that doesn’t work (and sometimes it doesn’t), then you’ll have to delete it instead.