Monilophytes = Leptosporangiate ferns, horsetails, whisk ferns, marattioid ferns, & ophioglossoid ferns.
According to A Dictionary of Entomology:
Latin, monil = necklace
As for "phyte," I'll quote this Source:
Greek, -phyte = denoting a plant or plantlike organism
So it literally means "necklace plants. "
From Rai & Graham (2010) :
The extant monilophytes (a name based on a “moniliform” or necklace-like stele thought to be ancestral in the group; Kenrick and Crane, 1997; Pryer et al., 2004)
Note : stele = the central core of the stem and root of a vascular plant, consisting of the vascular tissue (xylem and phloem) and associated supporting tissue.
In other words, the vasculature of all of these plants are arranged like a beaded necklace. This is further alluded to in Pryer et al. (2004) as quoted in @Kendall's answer