Sometimes I hear people say they don't like Music Theory. I think that it is important to have a concept, especially when you are playing jazz. There are lots of musicians who play totally by ear, and that is great if you are blessed with that strong of an ear. I think that an understanding of theory is a huge help, and it opens your ears to what you should be listening to. Regardless of what instrument you play, it is good to play piano to see what chords and voicings sound good with melodies. Then you can also play the chords and see what scales sound good with the chords. It is really helpful to trancribe a solo and analyze it to see what chords, scales, and notes are used. If you know the 12 major scales, and the chords from them, and the 12 melodic minor scales, and the chords from them, and how changing that one note (minor 3rd) changes the way the chords work, then you are 90% there. You might also learn the 3 diminished scales and the 2 whole tone scales. The 12 Blues scales are good to know. The rules for Blues are different then the rules for Western Harmony, but you can use all of these scales in jazz and your ear and the listener's ear will accept all of the dissonance.
If you learn all of these scales and how they work there are some other things in music theory that are good to know. Suspended chords are used a lot in folk music and in jazz during the 60's they were used more and more. The song Maiden Voyage by Herbie Hancock is all suspended chords. There are no chords in it that aren't suspended. The song So What uses chords that are voiced mostly in 4ths. This idea of voicing chords in 4ths also was used a lot in jazz in the 60's. Slash chords are where the bass is playing a note other that the normal root. They are written with a chord symbol and a / then the bass note, hence the name. Tritone substitution is another aspect of theory that it behooves you to understand. There is a thing where you place a chord over a tritone, and this is called Upper Structures, and that is another area of jazz theory that is fun to explore. A good place to use Upper Structures is in On A Clear Day and The Days of Wine and Roses.
Joe Pass said that a lot of people try to make their music too complicated, and he tried to simplify things. Mostly you are just dealing with Major, minor, or Dominant chords. The Dominant chords have the most potential for alterations, but mostly you would have 9th, Sharp 9th or Flat 9th, and one called the Lydian #11. Augmented chords and diminished chords can also function as Dominant chords.
If you are trying to play and you think about all of this stuff, of course it is going to slow you down. You want to get to where you don't have to think about it. But paradoxically, if you think about it a lot and really learn all of these scales and ideas, you will get to the point where you won't have to think about it. And you don't have to wait until you know everything before you even play. If you are at a jam session and you follow the protocal, wait your turn and don't try to play a million choruses before you let someone else have a shot, it will be cool. You might get a cymbal thrown at you, like Charlie Parker, but that only made him go back to the woodshed and practice harder.
Theory might be confusing at first, but it is like Algebra, and you start with basic ideas, like the 12 major scales, and build on them. Memorize this sequence: BEADGCF. The first 4 letters spell the word "bead" so that should help. If you keep adding them as flats you get the key of C, with no sharps or flats, then F, then Bb, and so forth. This is called the key signature. Now memorize that sequence backwards, and you have the sharps: FCGDAEB. The key of G has F#, the key of D has F# and C#, the key of A has F#, C#, and G#. It is like that Christmas song about the partridge in a pear tree. You just keep adding the sharps or flats one by one and each time you get a new scale. That is your first lesson in music theory, and you just build on it.