He was not one of the Twelve; but after the treachery and death of Judas Iscariot, someone was needed to take his place. Two candidates were selected, and lots were drawn to see which of them should be made one of the Twelve: the choice fell on Matthias. Nothing is known for certain about his subsequent history.
Drawing lots to select a candidate for an office sounds strange to us, but it was a recognised Jewish custom: for example, the priest who was to enter the Temple sanctuary and burn incense there was not chosen by some rota but by lot. Random events, independent of any obvious natural or human cause, were seen as a direct expression of God’s will. Drawing lots was not a substitute for human decision – human beings had chosen Matthias as a candidate, human beings decided which priests were eligible on which days – but a way of putting the final choice into the hands of God.
When we attain some high or responsible position, we may be tempted to congratulate ourselves on being the best candidate for the job. We would do well to remember that we have got there because of the people we have met and the things we have found ourselves doing, and, more fundamentally, because of the gifts and talents that God has given us. These things are essentially random: like Matthias, we have been chosen by lot.