The line of the fabric at right angles to the Selvedge. This the cut edge of the fabric when it comes off the bolt.
The crosswise grain of woven fabric has more stretch, and therefore the cross-wise grain usually goes around the body, and the lengthwise grain goes up/down the body.
Showing the lengthwise, crosswise and bias grain on a piece of fabric. The cut edge (crosswise grain) is shown with blue scissors.
Pattern pieces placed on the fabric.
As the crosswise grain of a woven fabric has more "stretch", it generally goes around the body. As can be seen in the pattern pieces laid on the fabric in this image, the stretch would go across the body; around the bust/back.
The stronger, lengthwise grain goes down the body, from neck to waist.