Two central images are used in Shakespeare's Sonnet 116.
Stanza two presents the image of love as constant as a star used by navigators to determine the location of ships. The image is an extended metaphor that makes up stanza two, and reveals love that stays constant through storms and is never shaken.
Stanza three presents the image of love's resistance to and immunity from time. Love is not time's fool or jester or clown, and love will not yield when time and the grim reaper try to bring death to love. Love will last until the final judgment. What love is not--the fool--is personified to form the first image, and is followed by the image of the grim reaper.
The enotes Study Guide on the poem explains these two stanzas as follows:
The second quatrain uses a series of metaphors to flesh out the character of proper love. Its constancy is such that it not only endures threats but actually strengthens in adversity. Its attractive power secures the beloved from wandering, and it sets a standard for all other lovers. Although conspicuous and easily identifiable, its value is inestimable. Aspects of it can be measured, and many of its properties are tangible, but it resides in another dimension, unassessable by normal instruments in space and time.
The third quatrain considers the constancy of true love under the threats of time and aging. It declares that love is unaffected by time. To begin with, love far transcends such mundane physical characteristics as size, appearance, condition, and shape. For that reason, it ignores physical changes caused by age or health. It defies time and everything in its power, including death. True love operates in the realm of eternity. Not even death can part true lovers; their union endures forever. Because love has the capacity to raise human action to this exalted state, it alone enables humans to transcend temporal limitations. Humankind becomes godlike through love.