Home field advantage refers to the advantage of a team playing in their own field.
In a cybersecurity context, organizations have control over their environments and have the luxury of time to shore up defenses, apply hardening settings, and other such actions to make life difficult for would-be attackers.
Defenders and administrators should know their networks better than an attacker who has no familiarity with it. As such, they should be able to act quicker and with more strategic advantage than an adversary attempting to inflict harm on their assets.