The "seven lost years" refer to two important periods in Shakespeare's life about which we know very little. The first period lasted from 1578—the year that Shakespeare left grammar school—until November 1582, when he married Anne Hathaway. The second period of so-called seven lost years lasted from 1585 to 1592. In 1585, we know that Shakespeare's twins, Hamnet and Judith, were baptized. But from then until his arrival on the London theater scene in 1592, Shakespeare vanishes from the record, and so we don't really know why or when he left Stratford-upon-Avon for London, or even what he was doing before he became a professional actor and playwright in the capital.
As one can imagine, the paucity of information regarding these two important periods of Shakespeare's life has generated a good deal of speculation, much of it ill-informed. A thriving cottage industry of myth-making has endured for centuries, spawning all manner of bizarre conspiracy theories, which it is generally advisable to ignore.