Vocabulary
Colla parte: To follow the tempo and phrasing of the soloist.
Solo ad lib.: To play freely in tempo and phrasing.
Harmony
The Bassoons solo suggests that the key of the piece is in C major. Then the French Horn comes in with a C# with is the major 3rd of the first note of the bassoons second measure, but a minor 2nd on the second note when the bassoon returns to C. This would then imply that we are not in C major rather we are in a freely tonal piece since from here on out there will be an excessive use of polytonality. You could say that Stravinsky's remark on these first three measures is foreshadowing of whats coming ahead.
Technique
The solo Bassoon is playing at its highest register which creates a pale and delicate timbre.