Why do most buildings and homes have two numbers on their facades? One is obviously the street number, but what is the other?

This is known as the číslo popisné, the descriptive or land-registry number. It is intended to be a more permanent indicator of a piece of land than the street address (known as the číslo orientační) which may change over time. The descriptive number is assigned by the town quarter and so is typically accompanied by the name (or number) of the quarter. In buying cottages, Czechs often look for places that have been assigned descriptive and orientation numbers, because this indicates that they are of a more permanent character than those issued only an evidence number (evidenční číslo).