That age is best which is the first,
When youth and blood are warmer;
But being spent, the worse, and worst
Times still succeed the former.                    - To the virgins, to make much of time.

Carpe diem is Latin for seize the day.
The poem-writer person, is telling virgins to quickly get married before it's too late. Because he thinks that youth is the best time of life, and the time after isn't that great. He stresses that virgins should get married in their prime, before it's too late and it'll be a challenge to get hitched.