Just as it says on the tin.
It will have the burnt prefix added to its name and take damage.
You can repair the damage, but the burnt prefix remains.
No effect on the item’s actual properties once repaired, however, just a name change.
Atleast in the version I have which is… 8259, from 5 days ago.
Pre-burnt items will burn faster if they are set on fire again (they will already be a stage in), but other than that, there isn’t much difference.