I am learning road code. There are 2 sentences I am confused about. Can you tell me what do they mean? Thank u?

Simple.
1) If you have no lane markings another vehicle may be coming the other way. If you can both stop in half the distance you can see then you avoid a collision.
2) If you can stop in the length of clear lane you can see then you will not hit anything stopped just out of sight.
It is all about driving sensibly to the conditions.