Let’s see if I can explain simply…a normal is the side of the facet that should face outwards - if it doesn’t, you don’t get correct light reflection from the mesh, hence artifacts. Sometimes in the modelling process, some normals get wrongly made and end up facing inwards.
OK - so what you do is select the mesh, go to the utilities tab and click on the one that looks like a square thumb tack. You get three options - select the A and you get a red arrow in the centre of each facet - they should all be pointing outwards, but it is difficult to spot the few which may not be. Click on the one that looks like a thumb tack with four spikes. That will give you a single yellow arrow. If it’s not pointing outwards, click on the A again, which should make it reverse direction. This show that your normals are unified and correctly aligned.
For some reason, I’ve never got the actual reverse normal option to actually do anything, but this method works for me.