Forster originally intended to title this novel Monteriano, after the small Italian village where Lilia falls in love with Gino. But his editor thought this title was too boring (we totally agree), so Forster changed it to Where Angels Fear to Tread.
This title comes from a poem called "An Essay in Criticism" by Alexander Pope. The full line in Pope's poem is: "For fools rush in where angels fear to tread," and sums up in a nutshell some of the key themes that Forster studies in his novel— the dangers of meddling in affairs when it's none of your business, and the dangers of rushing into, say, a quickie marriage with a hot Italian dude.
For more on the implications of the disastrous consequences that take place when characters "rush in where angels fear to tread," check out the What's Up With the Ending section.