The name Romford clearly derives in part from the ancient ford that crossed the small river that runs through the town.¹  There have, however, been a number of suggestions as to the full origin of the name:

One story says that the name came from when Queen Elizabeth I once visited the area and, on crossing the bridge said, "What a rum ford".  The town became known as Rumford, which changed, over time, to Romford.²  Sadly for this story the first mention of Romford is in 1153-4, pre-dating Elizabeth by around four centuries.³

Another suggestion is that the name is a contraction of Romanford, as the main Roman road from London to Colchester runs through the town.¹

The most likely source of the name, however, is the Saxon (Old English) for 'broad (roomy) ford'.¹

Interestingly, this means that the river Rom is named after the town, and not the other way round - that is to say, Romford is not 'the town at the ford of the Rom', but rather the Rom is 'the river that flows through Romford'.³

¹ Heritage Record, 1988, page 17.
² Romford History email group, message 612.
³ Victoria History of the County of Essex, volume VII, page 59.