What does "lobster" mean here?

Native speaker’s answer
Rebecca
To understand what Phoebe means by "lobster," you have to go back earlier in the episode. Ross receives a phone call from a man named Casey, who plans to go on a date with Rachel. Ross tells his friends that he wants to date Rachel. Phoebe then tells Ross that it's going to happen. He then asks how she knows that, and Phoebe tells Ross that Rachel is his lobster. She explains that lobsters fall in love and mate for life, and she believes that will happen to Ross and Rachel, that they will fall in love and be together for life.