A pictorial mark draws exactly what the brand does or deals in — a bird, a leaf, an arrow — as a recognizable, literal shape, in contrast to an abstract mark that owes nothing to any real object.
Its biggest advantage is that it needs no explanation: someone seeing the brand for the first time can guess roughly what it's about just from the icon. The tradeoff is a narrower well of material to draw from — competitors in the same category tend to reach for the same objects (a coffee cup for a café, a plane for a travel agency), which makes differentiation harder.
To survive at small sizes, the silhouette has to be simplified and the stroke weight kept uniform throughout. Rather than a detailed, realistic illustration, it usually reads closer to icon-style shorthand — a few lines and shapes standing in for the whole object.
When to use
Use it to communicate what the brand does without any explanation needed. Check first that the object you've chosen isn't already the go-to icon for everyone else in your category.