Advent means ‘the coming’ of something. Advent is a period lasting four Sundays (not weeks!) preparing for the celebration of Jesus’ birth.
The first Sunday of Advent also marks the beginning of the Church's year.
Because the incarnation is such an important event, Christians want to make themselves completely ready. In church they will hear readings from the Bible which they believe show that Jesus was the promised Messiah and the Son of God.
They may also be looking forward to the Second Coming of Jesus on Judgement Day at the end of the world.
To mark each of the four Sundays of Advent, a candle may be lit on a wreath (note the spelling, please!), and sometimes a fifth one on Christmas Day itself.