Jallow in the Gambia and Sierra Leone, elsewhere it takes the form Diallo. It's the surname of one of the four main branches of the Fulbe people (also called Peul, Pulaar, Fula, Fulani). They are a mainly grazier society, raising cattle and constantly moving to fresh grazing grounds. Thus they are found all over West Africa, from Mauretania to Cameroon.
The meaning of the name is unknown - The dictionary at
www.jtosti.com/noms (in French) mentions a Malinké word, Jala, meaning "turban", but without conviction. The Fulbe don't speak Malinké or any related language.
I have seen a pedigree showing the four branches of the Fulbe (Ba, Bari, Diallo, Sow) as descended from the four sons of a certain Ogba. In this pedigree the son from whom the Diallo/Jallow descend is called Diallubhe, so, if such ancestor legends can be trusted, Diallo may be a variant form of an ancestor's given name.