The wall in the middle is the texture itself. It is redrawn every time a value changes; drag to move it and use the wheel to zoom.
The Design panel lists the parts of the wall. Wall holds the bond and the shift between courses; Brick holds the width, height, corner radius, and bevel of one stone.
Brick carries two groups. Color holds the base color and the spread in hue, saturation, and lightness that makes each stone differ; Surface holds the grain, grain size, and mottle.
Joint sets the thickness, color, and depth of the mortar; Light sets the direction, the strength on the bevel edges, and the shadow in the joint. Weathering adds grime, moss, chipping, and fading.
The File panel creates projects and switches between them; each one stays in your browser. Export takes a file type and a scale and saves the image at the wall size times that scale.