Yog.0 is the name of an Anarch-compiled database of corporate server passkeys. Runners with access to the database can easily pass through low security code gates, as long as the relevant passkey is in the database.
The Yog.0 database is a crowdsourced compilation of sniffed, spoofed and logged passkeys. If the key to the gate is in the database, you're in. If not, change the gate!
Yog.0 is a reference to Yog-Sothoth, the "Outer God" cosmic entity created by American writer H. P. Lovecraft, as detailed in his short story, "The Dunwich Horror":
"Yog-Sothoth knows the gate. Yog-Sothoth is the gate. Yog-Sothoth is the key and guardian of the gate..."