After the Post Communion prayer, the Blessed Sacrament is solemnly carried to the altar of repose and the stripping of the sanctuary takes place.  The altar is a symbol of Christ (that is why we bow to it) and its stripping symbolises the stripping of his garments during his Passion until in the end all he had left were three nails and a Cross.  So the clergy and servers remove the altar frontal and cloths and other items such as candlesticks.  The stripping of the rest of the sanctuary of banners, lectern falls and so on, all remind us of the desolation of Jesus in his Passion.  A psalm may be sung while the stripping takes place.