It comes from the roman numeral chord designation system. Let's say we're in the key of A (a common blues key.) Each chord can be represented by a roman numeral
I ii iii IV V vi vii
A B# C# D E F# G#
The upper or lower case refers to the quality of the third in the chord, upper case = major third, lower case = minor third.
In a standard blues progression--one without any substitutions or additions or anything that is often done to blues to make it more jazzy--there are only 3 chords used. In any key, they are the I, IV, and V chords. In the key of A, the I, IV, and V are A, D, and E. A side note, they are all dominant chords but they don't provide a dominant function. What I mean is that they have the maj3 and min7 that makes them by definition dominant, but they don't always resolve to a I. (The function of a dominant chord is to pull back to a tonic chord, they aren't being used that way in blues.)
Anyway, to simplify, a basic 12 bar blues, in any key goes like this:
I I I I
IV IV I I
V IV I V
As you can see the only chords are the I, IV, and V. This system is helpful because it can be applied to any key, just find out what the I, IV, and V of that key are and plug in. Incidentally, this can be used to describe all chord progressions that have some relation to key (don't try using this to describe giant steps) and it is a helpful tool for explaining chord progressions.