After contrasting sheep to non-sheep, Jesus makes a major promise for the sheep (AKA those who obeyed God's call to believe in Jesus).
First, Jesus gives the sheep eternal life (10:28). The promise of eternal life is repeated from John 3:16 and 5:24 -- it's an important one! In particular, Jesus talks about how they will never perish to the end (10:28).
But more notably, Jesus talks about how no one can "snatch them out of [His] hand" (10:28). The guarantee of no one snatching them out implies that their place with Jesus is secure and guaranteed. In particular, Jesus describes the promise of "eternal security" (more commonly known as "once saved always saved"). When we trust in Jesus for everlasting life, we do not have to worry about our eternity.
Jesus guarantees the promise of eternal security because he works with the Father. Jesus talks about how the Father "is greater than all" (10:29), which explains why no one can snatch from Jesus's hand or the Father's hand. In fact, Jesus proclaimed that "[He] and [His] Father are one" (10:30). Eternal security is a plan with the involvement of both Jesus and the Father. Even more reason to find hope in Jesus.