Redstone is for crafting and brewing. Redstone can be obtained by mining redstone ore with an iron pickaxe or better, by trading with priest villagers, or by killing witches. Redstone can be placed on the ground to make redstone wire, can be crafted into numerous redstone components and devices, or used in brewing potions.
Redstone Ore can be found in small veins, and each ore contains 4 or 5 dusts. They are pretty common deep under ground and are usally useless in vanilla.
Redstone has various uses in the Hexxit mod pack.
It is used in Detector recipe from Artifice, Typesetting Table from the Bibliocraft mod. Meteor Shield, Meteor Timer, and Meteor Proximity Detector from the Falling Meteors Mod. Lunar Clock from Harken Scythe. Redstone Rock Candy from Legend Gear. Jasper Block from Project Zulu. Camo Gate, Torch Lever, and Secret Redstone from the Secret Rooms mod. Redstone Brick, Drawbridge, Igniter and a new recipe for the vanilla Clock added by Tinkers' Construct. Carminite, Reappearing Block, Encased Smoker, and Encased Fire Jet from Twilight Forest. Condensed Solvent, Elixirs, and is used in many of the Tome of Alkahest recipes from Xeno's Reliquary.
Tinkers' Construct also added new Clock Recipe