Lent is part of the Christian Calender, for the six weeks before Easter.
It starts on Ash Wednesday and runs until Maundy Thursday, and commemorates Jesus' 40 days in the wilderness.
Marking Lent is something that you do from Monday to Saturday throughout Lent. Sunday is always a day of celebration as that is when Jesus rose from the dead. This is how you square the 40 days into 6.5 weeks.
There are different Lenten traditions depending on which branch of the church you identify with.
For Anglicans, Lent is a time of giving something up that is difficult, and at the same time giving away (Almsgiving). For me personally, my perennial Lenten abstinence is my beloved wine. I give that up, and then the money I save, I use for Almsgiving. We try to have a Lent project at church, and we will do something that enables us to give money away, for example, "Sandwiches for Lent" means that you have sandwiches instead of a traditional Sunday roast - the money saved goes to our Lent project.
Other traditions may be more prescriptive about what exactly you give up, eg meat (but this is meaningless to a vegetarian, so not in the spirit of Lent).
Giving up something that is really hard is a way of drawing closer to God.