what is a give way vessel and a stand on vessel ?

Seafarer
Seafarer

A give way vessel can be classified as a vessel that is directed by the COLREGs to keep out of the way of another vessel and avoid a risk of collision or a close quarter situation.

A stand on vessel would be the one which has been directed in accordance with COLREGs to keep it's course and speed while the give way vessel manoeuvres to keep clear of it and avoid risk of collision or a close quarter situation.

Rule 13, 14, 15 defines which will be a give way vessel and which will be a stand on vessel. 

Rule 16, 17 defines their roles in avoiding collision.