There are more warp threads per square inch than weft (or crosswise) threads, meaning that the straight grain has less stretch than the cross grain.
The straight grain is the grain most often used in garments. It generally runs up through the centre front of bodices, through the length of the sleeves or up through trouser legs. The grain runs perpendicular to the ground.
When you place your pattern on the fabric you align the patterns grain line (often marked with a straight arrow) with the straight grain.