System for Cross-Domain Identity Management (SCIM) is a standard protocol/API that allows users and groups to be automatically provisioned from identity provider that support SCIM, to your Atlassian application. SCIM is supported by most major identity providers, including but not limited to Azure AD, Okta, OneLogin etc. Kantega SSO primarily supports SCIM version 2. The older SCIM v1.1 legacy protocol is only supported and tested for use with Okta's on-prem provisioning agent.
In brief, SCIM works by having the identity provider send provisioning instructions to registered applications whenever changes occur. As such it can be a far more efficient way to synchronize users than connectors, as only actual changes need to be transmitted, rather than full snapshots of directory state.