A trod, according to the OED, is a dialect word for a trodden way, a footpath, path, or way.  Brockett’s Glossary of North Country Words in Use of 1825 describes it as a foot path through a field. 
The word is still in use, having been revived by Christopher Evans from Scarborough, who has tramped across miles of field and moorland tracing the route of these old ways across the northern parts of the North York Moors. 
The essential feature of a trod is the single row of large stone flags, as seen on the cover of Evans’ book (pictured right) in an old photograph by Frank Meadow Sutcliffe, the Whitby photographer (1853-1941).  His photographs show a number of trods which have now vanished, and Evans believes that perhaps only 20% of them survive.
Nineteen “long trods” have been identified, some running from the northeast coast southwestwards; some following or crossing dales, such as Eskdale (both the north and south banks) and Glaisdale.  Two notable ones are:
* The “George Gap Causeway”: Blakey Ridge through Rosedale and Great Fryup Dale (a very steep descent, and trods can be very treacherous when wet) to Staithes;
* “Quakers’ Causeway”: from White Cross (just east of Commondale) over Commondale Moor to Guisborough; the present name is obviously later than the trod, which may have a monastic origin, leading as it probably does to Guisborough Priory.
But shorter trods are found all over the moors; they lead to individual farms, mills and churches.  And locally they converge on market towns (and former market towns such as Egton near Whitby, and Castleton in Danby Parish).
There is no single reason why trods were constructed.  They do not tally exclusively with the existence of monastic properties, though many may have such links, and it is probable that monastic labour helped create them – they are even sometimes referred to as Monks’ Trods.  Nor do they tally precisely with the locations of mineral workings (alum, iron or coal), or with the locations of crosses and other waymarkers (although some can be found on them, and many continue to be rights of way today). 
The key factor in any road’s existence, however, is to connect goods from their place of origin with the people who want them: the trods that lead from the coast provide one obvious example, connecting fish with local markets. 
The earliest ones date from mediaeval times, and they were still being constructed in the 18th century: Castleton, for example, did not develop until this later period.
Many trods are vulnerable by running alongside later highways, but the North York Moors National Park Authority is aware of their historical significance, as are some local history groups, and it is hoped that concerted efforts will be made to preserve those that survive. 

Source: talk given by Chris Evans to the Milestone Society at Hebden, April 2012.
RWH / April 2012