With SSO, an identity provider such as Microsoft Entra ID, Google Workspace or an in-house LDAP directory checks the login. The individual applications trust this provider and do without their own passwords.
The biggest benefit for small businesses is central control. When someone leaves, you disable one account and access to all connected tools ends. Multi-factor authentication can also be enforced centrally.
Many SaaS vendors offer SAML SSO only on higher plans. According to its pricing page, Slack includes it from Business+, Mattermost from Professional. Factor this upgrade into your cost planning. Our team chat comparison gives an overview.
SSO does not replace strong credentials at the identity provider itself. Always protect this central account with a second factor, such as an authenticator app or a security key.