Telling someone to have several seats means that the person has said or done something that you do not agree with. You can use this phrase when you are having an argument with someone or you strongly do not approve of what someone is saying or doing
When you say have several seats, you are basically telling someone to sit down and stop talking. You might hear someone using this phrase while watching American television, like a comedy or a talk show.
Remember that several means many or a lot of something.
A seat is a another word for a chair.
Your basketball team is not going to win the game this time! Have several seats!