HIV can adapt itself to the treatment you are taking. Then the drugs become less effective. HIV treatment with antiretrovirals often consists of a combination of several anti-HIV drugs. These all work in a different way to suppress the virus. Sometimes the HIV virus becomes resistant to a certain type of HIV drug. We call that drug-resistant.
There are several different drugs that have the same kind of effect on HIV, so your HIV virus can become resistant to all the drugs that have that effect. This is called cross-resistance.