What does the title of William Shakespeare's Sonnet 138 mean?

The answer above maybe doesn't have it translated into regular, non-poetic language, that people during Shakespeare's day, actually spoke. Shakespeare was just hard to get along with and wrote like this, maybe he was deslecsic, even though they had a better environment: When my mistress swears that she’s completely truthful, I believe her even though I know she lies, so that she’ll think that I’m some naïve young man who’s ignorant about the world and the tricks people play. I pretend to stupidly believe her lies while fooling myself into thinking that she thinks I’m young, even though she knows I’m past my prime. In this way, both of us suppress the simple truth. But why doesn’t she say she’s a liar? And why don’t I say that I’m old? Oh, because it’s easiest to love someone who seems to be trustworthy, and old people who are in love hate to hear their age discussed. Therefore, I sleep with her, and she sleeps with me, and we both flatter ourselves by lying about each other’s faults.