Right Side / Wrong Side

"Right side" and "wrong side" are just shorthand for which face of the fabric ends up visible on the finished garment. On a printed cotton the right side is usually obvious - the color and pattern are brighter and clearer on one face than the other. On a lot of solid-color fabric it's genuinely not obvious at all, and that's exactly when the label matters most, because sewing a whole garment together with one piece flipped the wrong way is an easy, entirely avoidable mistake.

Instructions almost always specify "right sides together" (RST) for a seam, meaning the two right-side faces are placed facing each other so the seam allowance ends up on the outside of that sandwich - which then becomes the inside of the finished, turned-right-side-out garment. Get it backwards and you end up with all your seam allowances showing on the outside, which is obvious enough once it happens that it's rarely a costly mistake, just an annoying one to unpick.

On fabric where I genuinely can't tell right from wrong by eye, I mark the wrong side of every cut piece with a small strip of removable tape or a chalk mark the moment I cut it - cheaper than discovering the mix-up three seams into construction.