Alpines are part of the ExtrabiomesXL mod. An Alpine is a cold, snowy, rocky region that bears a resemblance to Basins in vanilla Minecraft (stone patches on the surface), but is bigger, covered in snow, and is surrounded by mountains. There is usually at least one low part of the alpine mountain "wall" that the player can climb through. Fir trees can also be found near the mountain parts of the region.
Alpines are very vital to miners wanting to claim the riches of the underground. Diamonds, Redstone, and other ores can commonly be found around the biome. If you are searching for trees in a somewhat barren world, this is one of the places you can use to get wood, as there are fir trees occasionally growing around.