(at least 60% of total votes including YES + NO + Abstain
The constitutional threshold for these specific votes are 80% (security changes). See L44 celo-monorepo/packages/protocol/governanceConstitution.js at master · celo-org/celo-monorepo · GitHub.
Also the abstain votes are not factored in to get the final approval percentage. It is simply as @yomfana states:
Or here on L215: celo-monorepo/packages/protocol/contracts/governance/Proposals.sol at master · celo-org/celo-monorepo · GitHub