Apart from web interface amoCRM can be accessed via API. Such access request add-ons and extensions, mobile apps on iOS and Adroid platforms.
If the option “Filtering access to the API” is activated, then external services as well as mobile apps will not be able to access your amoCRM account.
To activate filtering access to API go to section Settings -> Security and then turn on “Filtering access to the API”. Don’t forget to click on “Save” to save the changes