Autologin means than when Debian runs a session starts without entering any password.
This is very comfortable, but it implies a security risk because if someone has access to our computer can access its content, so it would be very interesting for a public computer without personal data for generic use but not for a laptop for example.
In any case if we access by network, for example by ssh, the system will continue asking for the password.
To get this in Debian Stretch we modify the /etc/lightdm/lightdm.conf file and change the lines:
Simple and fast.