If you use putty then this is a familiar term. Visit here to see more about it:
https://www.digitalocean.com/community/t...t-to-a-vps
Short note: SSH key is an alternative to password. These days as we are advancing in technology our brute forcing power is increasing. If you log in to your server with a password then your server is vulnerable to bruteforce attacks. It can be cracked eventually.
A ssh key on the other hand is a secure key which only the owner has and no hacker can thus crack into your server as your server will no longer require a password which can be bruteforced.
Yes hackers still can enter but chances reduce. They still can Ddos to the server. If the server don't have any protection then yes your server can still be hacked.
Ssh key just reduces the options of a cracker but it doesn't stop them!