A Governance Action is a proposal that gets submitted On-Chain for voting. It is an on-chain event triggered by a transaction. Governance actions have an expiration period, after which the action cannot be enacted.
Any ada holder can submit a governance action for a vote on-chain. Once the action is recorded on the ledger, voters submit voting transactions.
In CIP-1694 describes 7 types of Governance Actions:
ga-new-constitutional-committee-or-quorum-size.md
ga-protocol-parameter-changes.md
Any Ada holder can submit a governance action to the chain.