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The slow movement, marked andante cantabile con variazioni, indicates a walking pace, songlike, in theme-and-variations format. Survivors of my Brahms series will recall that in the variations, the harmony underlying the theme must remain constant, bur everything else is permitted to vary; the format of theme is A-A-B-A’-B-A’. Beethoven writes this theme in E-flat; it’s a beautiful little tune reminiscent of Mozart, beguiling in its simplicity.

At 1:15, Variation 1 features an embellished piano line with violin and cello adding their comments.
At 2:25, Variation 2 goes to the violin and cello with the piano providing harmonic backing.
At 3:35, Variation 3 goes to the piano with the violin and cello playing pizzicato in the background. (“Pizzicato” means plucking the strings, not a small pizza!)
In theme-and-variation movements, it was practically a law that you had to write a variation in the opposite mode. At 4:40, Beethoven switches to E-flat minor for Variation 4 for cello and violin with the piano providing both pulse and harmony.
At 5:57, Variation 5 has the piano playing around the theme while the violin and cello work their own way around it.
At 7:10, he writes a coda that lays the theme to rest gently.

Beethoven: Piano Trio in C minor, Op. 1/3 (second movement)