so what exactly does it mean when you ask if someone's available to do H/MU on a shoot and the agent replies..

"____ is a 1st"
or
"____ is a 2nd that day"

The first could represent the lead on the job. The person who is the go to person and is in charge of the concept and final looks.   The second is the for lack of a better word, the assistant.  R-

"so what exactly does it mean when you ask if someone's available to do H/MU on a shoot and the agent replies."

"____ is a 1st" 
-On this date, you can put this artist on hold as you are the FIRST person to inquire about that date. That date on the calendar will remain holding for you until you confirm or release them OR the agent will call you before accepting any other booking on that date.

"____ is a 2nd that day"
-On this date someone else has put the artist on hold. If you accept the 2ND HOLD on this date, you will have this artist's availability only if something happens to the FIRST HOLD. Commonly, you would be the first person the agent would call if the other party cancels or confirms the job.

more simply:
1ST is AVAILABLE
2ND is UNAVAILABLE unless someone else cancels

Thank you Dan for bringing tenured wisdom to the dance here...
Greatly appreciated...

They mean the holds. First hold, second hold, etc. They are telling you what everyone's availability is. It's okay not to know things. Just say, "I don't know what you mean by that. Please tell me more."