Security tokens allow you to authenticate integrations such as your IDE or CI tooling.
Tokens can be generated by clicking your account icon at the top right of your screen.
Click My Account...
... and click Security.
Now you are on the Tokens page. The token you generate will only be able to be viewed once on generation. Make sure to store it somewhere secure (such as a password manager) or even better, add separate tokens for each integration to avoid trouble in the future.