What it is and where it comes from
Bone char is charcoal made by burning animal bones. Some cane sugar refineries, mostly in the US, use it as a filter to make sugar white. The sugar passes through it, and the bone does not become part of it. Beet sugar and most organic cane sugar are refined without it.
Where you find it
White and brown cane sugar, icing sugar, and sweets and drinks made with it.
What to look for on the label
It is never on the label. Most scholars and halal certifiers accept such sugar, because the bone char is a filter that has been fully changed by burning and nothing of it ends up in the sugar. If you prefer to avoid it, beet sugar, organic cane sugar or certified vegan sugar are refined without bone char.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.