Upon seeing the risen Jesus, the disciples, who used to feel afraid and sad, had their joy refueled by Jesus because his sudden appearance as a living person shook their perceptions. And now that their joy is rekindled, Jesus reiterated on their callings (since they are ready).
Jesus reassured them of peace and told the disciples that he also sends the disciples (20:21). Then Jesus calls them to receive the Holy Spirit (20:22), which is the ultimate helper. Many of his words are repeats of what he says in John 13 to 17, but this time, the disciples are equipped with joy.
Jesus also tells the disciples that "if [they] forgive the sins of any, they are forgiven them" and that "if [they] retain the sins of any, they are retained" (20:23). This verse is a bit tricky to understand; here's some notes that can help with understanding.
Obviously, the topic and purpose of Jesus's comment is the importance of forgiveness. According to Merriam-Webster, "forgive" means "to cease to feel resentment against". We know that the cross/resurrection is part of Jesus's plan of forgiveness and that Jesus's forgiveness clears us of our penalty of sin, and as noted in other parts of the Bible like Romans 3, we receive Jesus's forgiveness by accepting Jesus as Lord and Savior (which you can do right know). However, forgiveness is also a relational thing. When we forgive other people, we essentially do not let their past actions affect our present emotions in a very bad way (though note that precautions are based on knowing present heart and whether or not it changed). Similarly, not forgiving means letting the past affect us in a not so good way. Additionally, to an extent, proclaiming the gospel can be considered forgiving or retaining sins (as people either take or reject the offer).