I'm traveling to Prague and I noticed that every street has double addresses, one red and another blue. What's the story behind that?
The upper number is the "descriptive number" (Czech: číslo popisné or č. p.) and is unique within each municipal part (in this case, Nové Město, Praha 1).
Since descriptive numbers can be fairly large and since they're generally assigned based on age (i.e. newer houses have higher numbers), they're hard to use for navigation, which is where the second number comes in.
The lower number is the "orientational number" (Czech: číslo orientační or č. o.). It's unique for each street or square (in this case, Wenceslas Square) and can be used for navigation, since houses in a row should have increasing numbers based on their position in the street/square. Also, for streets, houses on one side of the street have even orientational number, and odd on the other side.
The same system is used in other towns and cities in Czechia. The plaques can have different colors, but their position (the orientational number below the descriptive number) should be consistent.
Side note: while the Czech names for these numbers are clear, I found different English translations. Some translate číslo popisné as "conscription number" and some translate číslo orientační as "reference number". I have chosen to go with the more literal translations.