TLDR

Volibear's chain lightning ultimate procs on-hit effects. For example, if your team has the glacial bonus, enemies hit by secondary chain lightning hits have a chance to freeze. Similarly, this works for on-hit items.

Full explanation

On-hit effects are effects that have a chance to proc on an auto attack. Volibear's chain lightning ultimate causes his auto attacks to be ranged and damages other targets in a chain. Each chain hit has a chance to proc an on-hit effect. This means that Volibear with many on-hit effects is extremely powerful (e.g. glacial, sword breaker, hush).