Fabric grain explained

Woven fabric has two thread directions. Cutting along them controls how a piece stretches, drapes and hangs.

Here the fabric's length runs left to right. In the planner it runs down the screen.

Lengthwise grain (warp) runs parallel to the selvages. It is usually the most stable direction, so most pattern pieces are placed with their grainline along it.
Crosswise grain (weft) runs from selvage to selvage. It usually has a little more give.
Bias is at 45°. It has the most stretch and drape. The planner doesn't do bias layouts: bias pieces need diagonal placement, and v1 only handles rectangles at 0° or 90°.

To place a piece on grain, measure from the selvage to both ends of the pattern's grainline arrow. The two measurements must match (example method). University extension guides call strict attention to grainline "one of the most critical factors" for professional-looking garments (University of Nebraska–Lincoln, HEG83-173 "Follow That Grainline"). The same series explains how to check and straighten off-grain fabric before cutting (G91-1028 "Preparing Fabric for Use").

Grain in the planner

Lengthwise: the piece's length runs along the selvage, upright (0°) or flipped end-for-end (180°).
Crosswise: the piece is turned 90° so its length runs across the fabric.
No grain needed (for example felt, some interfacings, or pieces where stretch doesn't matter): any 90° turn is allowed.
Unticking Allow rotation pins the piece to its first orientation.

Knits have a "grain" too: the lengthwise ribs (wales). Stretch direction matters more there, so check the pattern's instructions.