When read together, Sonnets 1-126 seem to be addressed to an unnamed young man that most scholars refer to as "The Fair Youth." Today, most scholars (like Harold Bloom and Stephen Booth) argue that we shouldn't try to read the poems autobiographically and we shouldn't assume that the figure being addressed in the poems is a real person. But, in the past, some literary critics (like E.K. Chambers) have argued that Shakespeare addressed sonnets 1-126 to a real life friend and/or lover. These are the two major candidates:
William Herbert, 3rd Earl of Pembroke: He was a younger patron of Shakespeare. (A patron is just someone who offers support and helps bankroll an artist.) His initials match the dedication of the Sonnets: "Mr. W.H.".
Henry Wriothesley, 3rd Earl of Southampton: This is another one of Shakespeare's patrons. Shakespeare dedicated two poems, The Rape of Lucrece (1594) and Venus and Adonis (1593) to Henry Wriothesley. That's why some people think that the "W.H" initials in the dedication of the Sonnets accidentally got flipped around and were supposed to read "H.W."