It means you bring about withdrawal. Eg if a person is addicted to drug A, and I give him an antagonist to it, it will precipitate the withdrawal signs and symptoms of drug A. He might still be taking the drug, but the fact that I've given him an antagonist means it isn't active anymore.
Opioids that are partial agonists will act as agonists when given alone. If a person is already taking a full agonist, these partial agonist opioids will act as antagonists, reduce the effects of the full agonist, and precipitate withdrawal signs and symptoms.
eg if a person is taking Heroin (full agonist) and you give him buprenorphine (partial agonist), the net effect will be a decrease in opioid action --> withdrawal signs.
If a person isn't taking any opioids and you were to give him buprenorphine, you'd just get opioid effects, not withdrawal.
