What is a salt in passwords?
In one sentence: A few random characters mixed into a password before it is fingerprinted, different for every person, so identical passwords look different.
A little more
When a website stores a password fingerprint (a hash), it first mixes in a salt: a short string of random characters made just for that account. The salt is stored next to the fingerprint; it isn't secret.
Because every account has its own salt, two people with the same password end up with completely different fingerprints. A thief who steals the database can't crack everyone with one guess or use ready-made tables, and has to start again for every single person. Salting is combined with deliberately slow recipes such as bcrypt or Argon2.
It's a bit like… Two people bake the same cake recipe, but each adds their own secret sprinkle, so the cakes never look alike.