Jamieson, Fausset and Brown on Exodus 30:18-21

18-21. Thou shalt ... make a laver of brass--Though not actually forming a component part of the furniture of the tabernacle, this vase was closely connected with it; and though from standing at the entrance it would be a familiar object, it possessed great interest and importance from the baptismal purposes to which it was applied. No data are given by which its form and size can be ascertained; but it was probably a miniature pattern of Solomon's--a circular basin.

his foot--supposed not to be the pedestal on which it rested, but a trough or shallow receptacle below, into which the water, let out from a cock or spout, flowed; for the way in which all Eastern people wash their hands or feet is by pouring upon them the water which falls into a basin. This laver was provided for the priests alone. But in the Christian dispensation, all believers are priests, and hence the apostle exhorts them how to draw near to God (Joh 13:10; He 10:22).