Here is possibly simple and good way for you to understand angles.
Throw a right hand punch in the air and then hold the position with the right hand extended.
If a person is straight in front of you, then its very difficult for him to strike you back as your right hand punch would be landing on his face.
However, if the other guy counter-attacked at a 45 degree angle on either your left or right side then your straight punch would have missed, but he could hit you in the face or body with a punch. The other guy could also attack your legs or body with a kick.
2 guys standing in front of each other and punching each other in the face requires LESS skill to do.
If 1 guy moves to the side and punches the other guy in the face, he gave himself an advantage. You could not hit him, because he was not straight in front of you. BUT, he could hit you, because he moved slightly to the side of you.