In jazz, and other more harmonically complex genres, it is very important to understand that a 7 chord (usually the 5th chord in a Major or sometimes Minor key) can be substituted with a 7b9 chord. This understanding unlocks the next very important concept of diminished substitutions (the next lesson in this series).
Let's examine some 2- 5 1 chord progress below that use both the 7 and the more complex 7b9 substitute for the 5th chord.
2- 5 1 in G
2- 5(7b9) 1 in G
2- 5 1 in C
2- 5(7b9) 1 in C
As you compare the 2- 5 1 progressions to the 2- 5(7b9) 1 chord progressions you should here the voice leading of the b9 of the 5th chord resolves one half step downwards to the 5th note of the 1 chord. This pleasing half-step walk down, and the addition of the more complex harmony, can make this progression a great tool.