Hatch is an area (the Hatch Valley, and the village of Hatch) that grows New Mexico green chile (no, they're not jalapenos--they're their own thing, and a specific type, of which there are several varieties).
The green chiles are sharper in flavor and have more front heat, while the ripe red ones tend to have a more complex flavor and less front heat but more back heat.
In the late summer and early fall, the chile harvest begins. You'll start to see crates of Hatch chiles showing up in grocery stores, and there will be chile roasting areas, too, where if you get a certain amount, you can get them roasted for free.
If you can't get to New Mexico to get some fresh roasted chile or to the Hatch Chile Festival in early September, Hatch does sell it online.
|Picture from Hatch Chile Co website|
If I have a recommendation from their website, it's to get the Sadie's Not As Hot salsa. Sadie's is a restaurant started in Albuquerque (they have a few locations) and the food is delicious. Their in-house salsa has a good kick to it, and their Not As Hot salsa does as well, but not as much.
If you're ever in New Mexico, the chile is a must.