Hi! Alberti Bass is a left-hand keyboard accompaniment pattern popularized by Domenico Alberti from the Classical period, where the pattern goes low, high, mid, high. An example in F major would be F, C, A, C. I often just see it as root, 5th, 3rd, 5th.
This group of four notes is repeated in the left hand while a melody is played in the right hand. I hope that helps!