"用板盖上" can be translated into English as "cover it with a board" or "cover [the object] using a board".
Here are a few examples to illustrate its usage:
Please cover the opening with a board to prevent dust from getting in. （请用板盖上那个开口，防止灰尘进入。）
He used a board to cover the broken window. （他用一块板盖上了那扇破窗户。）