Answer:
A first impression is your first thoughts about someone. For example if you saw someone how was dirty and smelly and rude to you your first impression of them is that they are not that kind and not much of a person you want to know. You never get a first impression again, so if that smelly, dirty person got clean and wanted to be friends with you, you would still think back to the rude person who used to smell.
Your first impression therefore is your last impression.
Another word for impression would be register- if something registers with you, it means you have noticed it and will probably remember it later.