"That's called a microphone. It's a big sausage that picks up everything you say - and you're starting early"

Prince Charles is making a joke about the microphone. He is indicating that the microphone resembles a huge sausage, and it picks up whatever you state. He is likewise indicating that the individual he is speaking with is beginning to talk early, which might mean that they are talking excessive or too soon. This might be analyzed as a cautioning to the individual to be careful about what they state, as the microphone will select it up. Prince Charles is likewise making a joke about the size of the microphone, as he is comparing it to a sausage. This could be interpreted as a lighthearted way of warning the person to be careful about what they say.