Alex says that the secretary will reserve a conference room for next Tuesday's meeting.
"To reserve" (reserve) means to arrange for something to be available, usually for a future date and time.You might need to reserve a hotel room so that you know there will be a place for you to sleep.
"To reserve" means to call or to contact someone and say, "I want to use this in the future; on this day and at this time."In this case, the conference room is being reserved for next Tuesday.
A "conference room" is a small room in an office building, usually with a large table (and many chairs) that you sit next to for your meeting.We would actually use the expression you "sit around the table."
Everyone is sitting next to the table, and that would be a conference room, or in a conference room.When Alex says that the secretary will reserve a conference room, then, he means that the secretary will arrange to have their meeting in a large room next Tuesday.
Alex says that everyone will be apprised of the location as soon as it is known."To apprise" (apprise) means to explain, or to tell; it could also mean to describe something so that you understand it.