What it is
Every token has a way to be created. On Solana, the mint account for a token stores a field called the mint authority. The documentation describes it as the account authorised to create new units of the token, increasing the supply. If that account holds the authority, it can issue more tokens at will.
Why it matters to holders
The supply of a token decides, in part, what each unit is worth. If the issuer can mint without limit, your share of the total can be diluted at any time. A stablecoin issuer legitimately needs mint authority to issue new tokens against deposits. A memecoin with an active mint authority held by an anonymous account is a bigger warning sign, because the creator can print more and sell them.
Revoking it
An authority can be set to none. Once the mint authority is revoked, nobody can mint more of that token, so the supply is permanently fixed at its current value. That is the property people mean when they say a token has a fixed supply on Solana. Verifying it means reading the mint account on a block explorer and checking that the authority is empty.
Not the only power to check
A token may also have a freeze authority, which can freeze token accounts. Having no mint authority does not mean the issuer cannot restrict transfers. When you assess a token, check both powers, who holds them, and whether the issuer is a known organisation with legal and reputational stakes or an anonymous account. A supply that is fixed by code is stronger than one fixed by a promise.