Warp vs weft
Warp yarns run the length of a fabric, parallel to the selvedge, and are held under tension on the loom. Weft yarns (also filling or pick) run across the width, inserted between the warp threads by the shuttle, rapier or jet. Warp is usually the stronger, more tightly-twisted yarn.
A fabric behaves differently along the warp and the weft: warp direction is stronger and stretches less, weft direction has more give. This is why cut direction matters in a finished product, and why construction is always written warp-first.
When you order from a mill
When you send a target construction, the first two numbers are the warp and weft counts, and the reed x pick is ends (warp) x picks (weft).