Figurative language is a little tricky, but think of it this way--if it's not real, it's figurative. So, in this case, if there's not really a funeral marching around a heart, it's a figure of speech. Figurative language is intended to draw a picture or create an image stronger than merely stating the reality. Types of figurative language include some things I'll bet you've studied or at least heard of: simile, metaphor, personification, hyperbole, and more.
In this line from The Crucible, the image is personification--giving human qualities to something non-human or non-living. A funeral is not alive, thus it can not march; that's what makes it figuragtive. John is telling Elizabeth that all the dread and dreariness and darkness of a funeral have settled around her heart, making her sad and mournful and however else one might feel at a funeral. The word everlasting is a little bit of hyperbole (exaggeration used for effect), since a funeral and the mourning that generally accompanies it will usually end. This marching funeral is everlasting--it just keeps going.
In Act Two of The Crucible, Elizabeth once again brings up Abigail, questioning whether Proctor would hesitate as much if it wasn't Abigail who he must "go to hurt." This is a sore spot in their relationship, as Proctor had engaged in adulterous activities with Abigail, breaking his marriage vows of fidelity to Elizabeth.
Proctor gets angry at Elizabeth for bringing up the issue, claiming that Elizabeth forgets nothing and forgives nothing and that "an everlasting funeral marches around [her] heart." As the previous educator has noted, this is an example of personification; Proctor is ascribing human action to a non-living entity.
In addition, this phrase is also a metaphor. Proctor does not literally mean that there is a funeral marching around Elizabeth's heart. Rather, he is referencing an activity associated with grief and death in order to suggest that Elizabeth is in a perpetual state of dread or mourning over the integrity of her marriage, her status as a wife, and her own pride. Referring to this kind of grief as a "funeral" renders it hyper-dramatic and serves as a way for Proctor to lash out at his wife, who he feels resents him and continues to punish him for his transgression.