Bardic songs last for one or more rounds and end with a final verse that carries a big payoff. Although magical, bardic songs don't count as spells; they don't force the bard to suffer opportunity attacks from engaged enemies, and they can't be canceled by effects that can cancel spells. In fact, bards can cast spells while in the middle of singing a bardic song.
Each song specifies what type of action starts it. To sustain it during the next round, it requires an action and a d20 check against its sustain target. If successful, the song can continue with its sustained effect for that round. (The next round will require another sustain check.) If your attempt to sustain a song fails, the song's final verse effect resolves immediately, and then the song's power ends. You can start another song on your next round.
You don't have to try to sustain the song at the start of your turn. If you choose not to sustain a song, its effects end immediately and you choose whether to use the song's final verse effect in the current round or to start a new song. You can't do both.
Some songs have an immediate effect that happens each time you start or sustain the song. Others have effects that continue throughout the entire round.
Most songs stop when a bard is knocked unconscious, silenced, or stunned. Having your song stopped this way prevents you from getting the final verse effect.
You can only sing one bardic song at a time. If you are singing a song (or spend an action to try to sustain a song), you can't start another song that round.
Bardic songs are loud, and cancel any of stealth effects you may have.
Your bardic songs don't stop immediately when you are knocked unconscious, stunned, or silenced. Instead, they continue for one round, giving you the chance to sustain the song on your next turn.