Main Building
noun
One of the meanings of 本 is main (which is what we're using here). The main public building is the main building.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あれ、本館にはどうやって行けばいいんだっけ?
Oh, I forget, how do I get to the main building?
じゃあ、本館の入り口に九時で!
Then, let's meet at the main entrance at nine o'clock!
トイレは本館の1Fにあります。
Restrooms are located on the first floor of the main building.