The chord progression 2 5 1 is the most common chord progression in jazz, it simply means the chords built on the 2nd, 5th and 1st notes of the scale as illustrated below first with Roman numerals and second with Letter names.
The 2 5 1 progression is an example of a Cycle Of 5ths progression in which the root note (the note on which the chord is built) goes down five letter names to the root note of the next chord. This is the strongest chord progression in tonal music and and in time its significance will become apparent. Let's now get back to improvising but this time over the 2 5 1 progression. The notes chosen in the improvisation must work with these chords and Im going to explain three different ways of tackling this. One is to use appropriate modes and scales (this may sound complicated but modes are just Major scales but starting on different notes). The modes for 2 5 1 in the key of C Major are shown below, Dorian for D minor7 (C Major scale starting on D), Mixolydian mode for G7 (C Major scale starting on G) and a C Major scale for C Maj 7.
Another thing that you can do when improvising over a 2 5 1 progression is to play broken chords not just in root position but all inversions. You can also create more interest by using approach notes from above and below the cord notes. I demonstrate all these techniques in the video below and also how to use shell voicings in the left hand.