Where the bias runs
Woven fabric has threads running along its length and across its width; that is its grainline. The bias is the diagonal between them, at 45 degrees to the straight grain.
Why it matters
Fabric cut on the bias has more stretch than fabric cut along either grain. That makes bias strips good for binding curved edges, because they bend smoothly around the curve.
The same stretch can work against you. Edges cut on the bias, such as the long side of a triangle, can stretch out of shape if they are pulled while sewing or pressing.