How we cite our quotes:
…love is not love
which alters when it alteration finds,
Or bends with the remover to remove. (2-4)
People who really love each other always will, even if one or both partners change, or if someone tries to come between them.
O no! [Love] is an ever-fixed mark
That looks on tempests and is never shaken (5)
True love isn’t affected by "tempests" like arguments or other difficulties; this also implies that forgiveness, in the ideal world the poet invokes here, is totally complete.
Love’s not Time’s fool, though rosy lips and cheeks
Within his bending sickle’s compass come,
Love alters not with [Time’s] brief hours and weeks,
But bears it out even to the edge of doom. (9-12)
True lovers are eternally faithful to each other, perhaps even after death. The "edge of doom" mentioned here could either be seen as the end of an individual’s life, or the end of all life in this world or the next – Judgment Day.