Lent is a certain time period leading up to Easter, usually 40 days. This custom is practiced a lot by Catholics and other Christian churches but can really be a great time for all Christians to reflect on the life of Christ leading up to his death.
Here are some good links to get a better understanding of Lent:
Wikipedia Entry for Lent
This is a great overall page with a lot of general information. Good starting point.
For most, this Wednesday (March 9) kicks of Lent with Ash Wednesday. Christians will attend church and ashes in the shape of a cross put on their forehead. This article has a nice explanation, which starts in the second paragraph.