A skirt co-ord is two pieces bought to be worn as one, which sounds obvious until you realise how much of your wardrobe is actually a negotiation between a top that nearly matches and a bottom that almost goes. The whole point is that someone else has already done that negotiating for you. You put on the set, the colour or print carries straight through from waist to hem with no join, and you look like you planned something when really you just got dressed. I own three and I've never once stood in front of them wondering what to wear with them, which for a self confessed overthinker of outfits is its own kind of holiday.
The clever bit is that you don't have to wear them as a set forever. Buy one with a decent skirt and a plain enough top and you've actually bought two separate things that happen to also work together, which is the only way a co-ord justifies its existence on a real budget. The skirt goes with a jumper when the weather turns, the top goes with jeans the next weekend, and the set itself comes back out whole for anything from a work do to a lunch where you want to look like you tried without announcing it. Spring and early autumn suit them best, when you're not fighting the top with a coat that hides it anyway. On fit, go up a size in the skirt before you go up in the top, because the top is usually the forgiving half and the skirt is where a waistband either sits properly or digs in and ruins the line of the whole thing. Check the skirt isn't cut so narrow at the hem that it stops you taking a proper stride, and if it's got a side zip, do it up and sit down in the changing room before you decide it's comfortable.
Where they go wrong is almost always the matching itself. A good one is dyed as a single batch so the colours genuinely line up, not printed in two separate runs that are close enough on the rail and obviously not once you're standing in daylight. Feel the fabric of both pieces, they should match in weight as well as colour, because a floaty top over a stiff skirt looks like two different outfits that got married. Check the seams on the skirt sit flat rather than puckering, and look at the waistband finish, a cheap one will fold over on itself by the second wear.
Reach for it on the days you haven't got the energy to make two decisions, only one, and want to leave the house already matching.