It had previously been rumored that the phone would be called the iPhone 5SE, to indicate that it's a "special edition" of the 5S, but 9to5Mac says that Apple believes the name would be too confusing. It's also possible that Apple simply thinks it'll be harder to sell a model with "5" in the name at a time when the iPhone 6 and 6S are around.
The iPhone SE is essentially supposed to be an iPhone 5S with much improved specs. 9to5Mac reports that it'll have an A9 processor, like the iPhone 6S, an improved 8 megapixel camera, like the iPhone 6, as well as a fingerprint scanner for Touch ID and NFC for using Apple Pay. There are reported to be minor physical changes to the body, including slightly curved edges along the phone's glass and a power button off to the right. The phone is expected to be announced at an event on March 15th, alongsidenew Apple Watch bands and a smaller iPad Pro.