In a very brief nutshell, Lent is a religious holiday that is most popularly observed by Catholics, but many Protestant denominations practice as well (I am a Protestant Methodist, for example).
Lent is the 40 days (not counting Sundays) between Mardi Gras (also called Fat Tuesday, the day after Fat Tuesday/Marti Gras is called Ash Wednesday) and Easter. It represents the 40 days that Jesus spent in the desert abstaining from Satan's temptations. Lent is a way to pay tribute, in a way, to that strength and sacrifice that Jesus made. Lent ends on Easter, which is a huge and joyous celebration of Jesus's conquer over death when he rose from the grave three days after being crucified.
It is practiced differently depending on the church. Some churches take it very seriously, some recommend Lent to the congregation, and some churches do not really talk about it at all.

Hopefully that was an OK explanation. I'm no expert or anything, this information is just based on my own experience