Say you're at a party, standing with your partner. Someone comes up and starts talking to them. A lot. The person is giving your partner focused, intense eye contact and enthusiastically asking questions. (Remember, flirting doesn't have to be explicitly sexual.) You're left in the shadow of the conversation and feel jealous as a result. Perhaps you feel that the person is sexually interested in your partner and that your partner's response could be sending the wrong message. Perhaps you wish, on some level, that the other person was giving you attention instead of your partner. This is totally natural.
So what to do about it? Well, if you can, in the context of the conversation, pipe in! Odds are, your partner will pick up on your hope to shift gears. If not, wait it out, and explain how you're feeling once the other person has left. Plain and simple, admit to your jealousy: "Hey, I felt kind of jealous when X came up to us at the party. I felt like they were giving you a lot of attention, and I felt left out." From there, you can hash it out and clarify expectations.