I'm learning "Recreation" on my piano. I've come to those little notes, and I have no idea what they mean. I can't play them, as they are too far away from each of my hands. May anyone help me ?
Carlo Castellani himself doesn't play them, and there several other differences between his performance and the score you linked to - including some more 16th-notes on the last beat of bar 43 which your question is about.
I would say this is just a mistake by the transcriber. Possibly, he/she put put those 8th-notes in the score as a "guideline" before filling in all the 16-notes, and forgot to delete them - such things can happen!
You already linked to Castellani playing the piece in your comment on another answer, but comments can be deleted without warning, so I put the link in my answer to keep it for posterity:
I think this is a transcription of a soundtrack, not a piece composed specifically for piano?
He's indicating that the melodic line around middle C existed in the original, but admitting that it can't be played by a two-handed pianist.
Still useful information though. Perhaps some would choose to 'perform' this piece on a sequencer, when those notes could easily be included.
Probably a piano score for Glenn Gould, to indicate what he should be humming if he can't contain himself. Basically, it is the line that the piano play is supposed to insinuate (note that it's an octave or two below the lower notes of the sixteenth phrases). If there is any melody instrument playing along with the piano, this would be its track.
And if this is played by a chromatic button accordion, it actually fits in the right hand (assuming that you interpret the 8va in the upper staff to hold for the small notes as well).
My main guess, however, would be that the transcriber proposes how an additional melody instrument could pick a lead to play, even if the original rendition is piano only.
It could also be what some other melodic instrument is playing, as a cue for the pianist.