Two-factor authentication (2FA) adds an extra layer of security to your Slack account. Once you enable two factor authentication, you will be prompted to enter a verification code in addition to your normal password whenever you sign in to Slack. If you’re a member of multiple teams, you will need to set this up for each account you own. You can find the link to enable 2FA for each of your accounts below
Download and install an authentication app on your phone or tablet. Slack 2FA will work with most time-based, one-time Password (TOTP) applications. Note the Slack beta app on the Windows phone does not support 2FA.
On the Settings tab, click “expand” next to “Two factor authentication.”
Enter your password, then click “Enable two factor authentication.”
On the 2FA configuration page, enter a 6-digit verification code generated by the app, then click “Verify Code.”
For more information visit: https://slack.zendesk.com/hc/en-us/articles/204509068-Enabling-two-factor-authentication