Hellstone Bars are made from 1 Obsidian and 3 Hellstone at a Hellforge. They are used to create Hellstone Tier items. Items made from Hellstone Bars are the highest quality craftable pre-hardmode items that require bars.
To craft the entire set of Hellstone tier items it takes 140 Hellstone Bars, which equals 140 Obsidian and 420 Hellstone.
When you mine Hellstone, it leaves half a block of Lava behind it. Therefore, it's imperative to divert the lava flow first if you don't have anything that could prevent lava damage. If possible, make a hole in the ruined houses may work.
A challenging but worth-the-work way to get the obsidian for these items is to drain your ocean down to the Underworld, or obtain the Bottomless Water Bucket from Angler Quests (Hardmode only, 2.5% chance).