The monstrance is a standing vessel with a window at its centre, made for a single purpose: to display a consecrated host so that it can be looked upon and adored.
Adoration made visible
Where the chalice and paten hold the sacrament during the rite, the monstrance exists for the devotion that grew up around gazing at it outside the rite, the exposition and adoration of the Blessed Sacrament and the carrying of it in procession, above all at Corpus Christi. The consecrated host is held upright behind glass in a central chamber, the lunette, so that the faithful may see it, which is the whole function of the object and the reason it developed only as that devotion took hold in the later Middle Ages.
The sunburst
Most monstrances are built as a sunburst, with gilded rays radiating from the central window, an image chosen to present the host as a source of light and to draw the eye to it. The form can be elaborate to the point of being a major work of the goldsmith, since the object is made to be seen and honoured. Its use is confined to Catholicism and a few High Anglican settings, since it presupposes the belief that Christ is really and permanently present in the consecrated host, and it is that belief, more than the object, that the sunburst is designed to proclaim.