The checklist for my Mongolian visa application mentions
Copy of the bio-data page of your current passport (including passport of other nationality)
What's a bio-data page?
The bio-date page of a passport is the page with your biographical data (name, date of birth, passport number, expiration date, etc...). Most passports will also show your picture on this page, along with the machine readable zone at the bottom.
"Bio" invariably either refers to biology (e.g., biochemistry, bio-informatics, biodegradability, bio-weapons) or is an abbreviation for biography (used on its own or, e.g., in biopic). In the context of travel, it's also the IATA code for Bilbao airport.
In this case, only the second makes sense: it's asking for the photo page of your passport that has your personal information on it.