An altar pentacle is any kind of plate, tray, or tile onto which is drawn, painted, carved or inscribed a five-pointed star. This can be permanent pentacle like those offered here, or it can be an object onto which you draw the pentacle temporarily. 

An altar pentacle is used when blessing or empowering anything on your altar. You can use it to bless salt, water, incense, or to burn spell candles on.