Texture is the arrangement of 3d forms such that they follow along the surface of another structure. Meaning, as the parent structure moved, deforms, etc. the textural forms along its surface follow with these changes.

Where a pattern would just be flat - like a design on a t-shirt, or like regular flat wallpaper that may have shapes and colours featured on it, but ultimately cannot be felt as anything but a flat surface - texture is three dimensional, and can be felt with your eyes closed.

This section from the beginning of the texture material in Lesson 2 covers this in more detail.

Generally in Drawabox, texture can be explained as showing how a specific surface would feel like to touch. Everything has a texture, be it coffee beans or clouds Each texture has its own pattern, but not every pattern is a texture, e.g., color patterns are not a texture we consider in drawabox.

If something casts a shadow onto its surface, it can be felt by touch. Roasted coffee beans have bumps and creases, and a dented line in the middle, those are textures you can draw. 

In a pile of coffee beans, each bean casts a shadow onto another, you can draw that cast shadow of the bean. 

Clouds might be a little bit difficult to turn into texture, but they have their own cast shadows as well in their shapes.

Hope it helps!