Every business has a footprint facing the public internet: your website, your email, remote-access services and anything else exposed to the outside world. Attackers start there, because it's the part of you they can reach without any help.
An external attack-surface test looks at that footprint the way an unknown attacker would — mapping what's reachable, what's running, and where the weak points are — so you find and close the gaps before someone with bad intent finds them first.
We're expanding this into a full guide.
In the meantime, see Penetration testing & vulnerability scanning, or talk to us about your setup.