Store Sign
noun
A watch over board is a board that goes over your store and watches over everything. What board on a store watches everything? It's the one that gets the most visibility, so it's probably the store sign.
かんばん
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. Be careful of the はん changing to ばん.
酔っぱらいの男が殴ったせいで、お店の看板がへこんでいるんです。
There is a dent in the store sign because some drunk guy punched it.