Bitumen drops from Bitumen ore, and is part of the Utility Ores Module. In previous builds, it seemed that it was able to be used as a fuel source in itself, but the bug has since been fixed in newer updates.
When cooked in a furnace, it yields Tar, which is an alternate for slimeballs. This means you
can use tar in the place of recipes that require slimeballs, such as sticky pistons
or magma cream.