It describes something that has the qualities of a biscuit, such as:
* Texture: Crumbly, dry, slightly sweet
* Flavor: Usually buttery or slightly sweet
* Appearance: Often golden brown and slightly rough
Examples:
* "The dough had a biscuity texture."
* "The cookies had a distinct biscuity flavor."
* "The cake had a biscuity crust."
You could also use "biscuit-like" or "biscuit-shaped" to describe something with a similar form or appearance to a biscuit.