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A common envelope (CE) is a single layer of gas surrounding both companions of a binary star; essentially, the outer portion of a star (stellar atmosphere) is shared between the two. Such a star is termed a common envelope binary (CEB), which can occur as a phase in the life of some binary systems, the phase termed a common envelope binary event (CEBE).
The existence of common envelopes is basically theory, though there have been candidates. The concept explains the origin of some chemically peculiar stars, such a star termed a post-common-envelope binary (PCEB). The common envelope is presumed to last a relatively short time compared to the life of the stars, triggered by a runaway process during accretion from one companion to the other, producing a single object, externally like a red giant, but internally with two stellar cores. It is presumed the common envelope is soon ejected (likely even if the stars fully merge). The resulting transient is presumed to be brighter than a nova but not as bright as a supernova. The event (i.e., CEBE) would tighten the orbit of the two companions. Given the variety of possible characteristics of each of the stars (mass, age, which phase of stellar evolution they occupy), there is presumably a lot of variety in the outcome of such events.
Given the type of compact object mergers producing LIGO/Virgo GW detections, it seems possible that some or all of them earlier experienced a common envelope binary event.