In simple terms, Alberti bass is a way of playing broken chords following a certain pattern. It's based on the basic 3-note chord or triad and the notes are played alternately, starting with the root of the chord like so: 1st, 5th, 3rd, 5th.
That's all there is to it. Like so much of music, once you know the pattern, the rest is easy. Here's an example of the C chord written as a solid chord, followed by the Alberti bass version of the same chord. It's written out for the right and left hands so you can have a go at each one, but normally you'd play it with the left hand only.
