What does github actions environment variable "CI" stands for?

it's the environment variable defined by convention in many CI servers like github action, Gitlab, Travis etc. Its value is always set to true when builds run in those servers.

Many tools check whether that variable is set to true and adjust their configuration and output to be suitable for CI environment. E.g. some tools are interactive by default, but if CI=true, it will skip user prompt and use default values/passed in arguments. Another example is pipenv that has special configuration when CI=true: https://github.com/pypa/pipenv/blob/207f2f565d2c4493bd8cdfb55a9b565aa58984fc/pipenv/environments.py#L27