Part of our identity as followers of Jesus, is to get outside our comfort zone and our regular context. We are all called to take the Gospel with us beyond the places where we and the Church live. Mission is about sending people, as Jesus sent his disciples, into the places of the world where the church isn't already present in some way. This is the work that we do when we go on mission trips; in doing ministry in the public arena; through youth ministry out in the community; by offering public worship outside of our buildings. Mission is different than outreach, which is reaching out within the communities and contexts we find ourselves in. Supporting local organizations with money is a good form of outreach; mission involves relationship. When we form a relationship with someone outside our comfort zone and the day to day parts of our lives, we're probably engaged in mission.
For more on mission: http://archive.episcopalchurch.org/79425_130201_ENG_HTM.htm