Although many chords used in jazz consist of at least four notes (Cmaj7 alone usually has CEGB), and can be awkward to play, it isn't always necessary to voice every note in the chord. By reducing chords down to their bare essentials (usually the root, 3rd and extension) you can have chords which are easier to play, while still implying the full chord's harmony.
Let's apply this to that Cmaj7 chord I mentioned above to see how it works.
Cmaj7 is made up of the notes C E G B. On guitar we often change the of these notes, and sometimes repeat one or more of the notes in the chord shape we're using.
This repitition of notes can add fullness to the sound, but is not always necessary. Lets play a take away these extra notes and just play C E G B. (Chord #2)
Hmm.. it sounds kind of simple, and is still awkward to play.
Let's reduce it even further by taking out the 5th. The 5th usually adds 'power' or strength to the sound of the chord, however, it is not essential to the chord's harmony.
We have now reduced a Cmaj7 chord down to it's most basic and essential notes. It should be easier to play, and sounds great, especially when finger picked. These types of chords were used a lot by Freddie Green.