Keychain is a password management app for macOS. The Slack desktop app uses Keychain to store your account information. 

To grant Keychain to access Slack, click Allow or Always Allow (for fewer requests) when you’re prompted by macOS.

Note: Clicking Deny won't stop Slack from starting but, it may break features that depend on Keychain.

💡 More information on Keychain Access can be found in Apple's Support Center.