Mark 1:21–28 (ESV)
21 And they went into Capernaum, and immediately on the Sabbath he entered the synagogue and was teaching. 22 And they were astonished at his teaching, for he taught them as one who had authority, and not as the scribes. 23 And immediately there was in their synagogue a man with an unclean spirit. And he cried out, 24 “What have you to do with us, Jesus of Nazareth? Have you come to destroy us? I know who you are—the Holy One of God.” 25 But Jesus rebuked him, saying, “Be silent, and come out of him!” 26 And the unclean spirit, convulsing him and crying out with a loud voice, came out of him. 27 And they were all amazed, so that they questioned among themselves, saying, “What is this? A new teaching with authority! He commands even the unclean spirits, and they obey him.” 28 And at once his fame spread everywhere throughout all the surrounding region of Galilee.
Mark uses unclean spirit and demon interchangeably. The parallel passage in Luke says “an unclean demon” (Luke 4:33). Unclean refers to that which is polluted or contaminated and therefore ungodly. Satan and demons had marginal authority over the created order... until Jesus came and invaded the dominion of evil with the Kingdom of God. Jesus' victory over satan and demons, ultimately culminating in his defeat of death at his resurrection, isn't solely an illustration of his great authority and power. It also means power and liberation for humans. Humanity was formerly almost powerless against unclean spirits, now, through Jesus people can be delivered.
In light of the big idea of this passage, Jesus has authority over the devil and demons. Not only do they know who he is but they listen to him. It's not even a fair fight. Rooted in Jesus' power his disciples have that same authority over the forces of evil.
Note the discrepancy in the number. Jesus and Mark refer to the demon (singular) and the demon uses the plural "us". It could be that the victim is possessed by multiple demons as in Mark 5 or that the demon is aware of Jesus' mission, not just to defeat him, but to defeat the kingdom of evil.
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*Some of the things they say about the demons and the satan are not universally accepted interpretations. Much of the Bible's teaching on spiritual beings is subtle and difficult to come to hard and fast conclusions.