Citations: Matthew 18:10-14 || Luke 15:3-7, Matthew's setting is the community discourse on humility + restoration; Luke's setting is the Pharisees' murmur at Jesus eating with sinners. Main point: God / the Good Shepherd actively searches for the one lost, even at cost to the ninety-nine.
The parable
A man has 100 sheep; 1 strays. He leaves the 99 (Luke: "in the wilderness"; Matthew: "on the mountains") and goes after the lost one until he finds it. Carries it home rejoicing; calls friends + neighbors to celebrate.
Two contextual framings
Luke 15, Jesus answers Pharisees' grumbling. The parable says: God's heart is to search for sinners; rejoice when one repents; "joy in heaven over one sinner who repents, than over ninety-nine righteous people who need no repentance." (Luke 15:7).
Matthew 18, Jesus speaks to disciples about community life. Don't despise "little ones"; the Father is not willing that one of them should perish.