The item in question is called a Communion-plate. Its purpose is to catch the host (or some part of it) should it be dropped.
The General Instruction of the Roman Missal mentions the Communion-plate in the section concerning articles to be prepared for Mass: “The following are also to be prepared . . . the Communion-plate for the Communion of the faithful” (118).
Additionally, Remptionis Sacramentum encourages its use: “The Communion-plate for the Communion of the faithful should be retained, so as to avoid the danger of the sacred host or some fragment of it falling” (93).