These trees grow in the desert. They look to be completely dead but are not. Frankincense is the resin that oozes from wounds in the bark. The trees are scarred with a knife so that the resin oozes. It is left to harden and then harvested.
Frankincense is a highly perfumed resin used in incense or as an essential oil. It is supposed to be anti-inflammatory.
Here’s the resin.
Frankincense trees growing in a wadi.