Arlo has many possible derivations: possibly a shortened form of Italian name Carlo, in Basque-speaking regions it is the word for area, and in English, Arlo was first used in the 16th Century by Edmund Spenser in The Faerie Queene and is thought to derive from the Celtic word Aherlow (meaning between two highlands).
Arlo, strongly associated with shaggy singer Arlo Guthrie, has an animated and cheery feel, thanks to its upbeat o ending. Arlo was chosen by Toni Colette for her son, which might give it a further bounce in popularity.
Arlo was popular in the first half of the twentieth century--it was Number 667 in 1915, for example. However, it's been experiencing a bit of a revival and might even surpass its peak in popularity -- it was one of the fastest-rising names of recent years and currently sits at Number 539. Arlo is even more popular abroad, reaching Number 85 in New Zealand and 187 in the UK. Arlo is a great favourite on Nameberry, sitting at Number 138.