The phrase "How do I love thee" refers to a famous line in one of Elizabeth Barrett Brown's poems from her collection, "Sonnets From the Portuguese."
"Sonnets From the Portuguese" contains a selection of 44 love poems written in sonnet form. These poems were written during the period of time in which Robert Browning, Brown's soon-to-be husband, was courting her. Of the 44 sonnets, number 43 is perhaps most known and begins with the line, "How do I love thee? Let me count the ways." According to Poetry Foundation, number 43 was used by Brown to describe the depth of her feelings for Browning.