This in arabic is called a miswak (or siwak). The twigs are from a regional tree called Salvadora Persica. The bark is chewed off one end and the fibres resemble a brush and is used that way. Lots of studies have been done and results showed that apart from the physical action of brushing, there were natural anti-microbial effects aswell.

OK thats my egghead response after reading it up on the net :)