William Shakespeare wrote a total of a 154 sonnets, which were published together in 1609. This sonnet was the second in that bunch, and it's one of many that are addressed to a mysterious young man who is often called the "Fair Youth." No one can say for sure who this guy was, or even if he really existed, but many scholars think he was a young nobleman whom Shakespeare would have known.
Another important thing to know about Sonnet 2 is that it's one of seventeen that are called "the procreation sonnets." Procreation, as you might know, is just a fancy word for having babies. So this sonnet, along with the one that comes before it and the fifteen that come after it, are all about trying to get this young man to marry and have a kid.
Now maybe you've heard about parents nagging their grown children to get married, and maybe you've even had that happen to you. We imagine that things weren't all that different back in Shakespeare's day. The thing to remember here, though, is that if this particular young man were a nobleman, the stakes would have been a lot higher. When he got old enough, a man like this would have inherited vast wealth. To keep that wealth in the family, and to keep his bloodline going, he needed to produce an heir. What this means is that a young nobleman's plans for having kids were a big deal, and there were personal and political consequences if he didn't manage to pull it off.