Can someone explain RSA-PSS to me? I can't find any usable resources about it online (at least ones that make sense to me right now), so any links to articles or if you could explain it to me would be greatly appreciated.

What I don't understand is what is PSS? I understand that RSA is an encryption type with two keys (usually public and private), but is PSS a signature (like the name suggests) and if so, how is it different from a hash?
A fixed hash function can be computed by anyone.

A signature depends also on a public key, and can be computed only by someone who knows a secret related to the public key.  Anyone can verify a signature on a message under the public key, but only knowledge of the secret enables creating signatures.  Signatures practically always involve the use of a hash function internally, combining the hash output (and sometimes hash input) with the public key using obscure mathematical spells.

RSA is not itself an encryption algorithm; it is a family of trapdoor permutations, which are obscure mathematical spells not really useful in isolation.  There are encryption schemes and signature schemes both built out of the RSA trapdoor permutation family, such as RSAES-OAEP and RSASSA-PSS.