Twill weave is one of the basic weaving techniques which is found in basically every type of fiber and filament. The twill line is the diagonal rib running from the bottom left of the fabric to the upper right, and is created by weaving the weft over two and under two warp threads. The next line in either direction (up or down) move out by one, creating this diagonal effect.
In the image above, the blue lines are the warp thread and the white lines are the weft threads. Twill is a very stable weave, not particularly prone to snagging. Fairchilds Textile Dictionary says “twill weaves are used to produce strong, durable, firm fabrics (p. 643).” And this may be true of other fibers, but in silk, it’s flowing and drapes beautifully, with a limpid hand and liquid flow.