The reason why taking your HIV treatment properly (adherence) is so important is because poor adherence can lead to the development of resistance.

This is a term doctors often use. It means that the type of HIV you have is, or has become, resistant to some anti-HIV drugs. In other words, the drugs don't work well any more.
Every time HIV makes a new copy of itself, it is slightly different. Often these differences are not important but sometimes the new copy is different in a way that is 'resistant' to the drugs you've been taking. This means that it will be able to reproduce again, even when you take the drugs.

If you are taking combination therapy, it is important to make sure that enough of the drugs are in your blood all the time to do their job properly. Missed or late doses could mean there are reduced levels of the drugs in your blood. This could allow the virus to make more copies of itself, including drug-resistant copies.
Drug-resistant HIV could lead to the treatment not working, and you not being able to use the same drug (and, sometimes, other drugs in the same class) again in the future.

If you are able to take each dose of the combination therapy at the right time each day, then the development of drug-resistant HIV is unlikely. That means the drugs will work for many years.

If you do develop resistance to some drugs, don't panic - there will still be other treatment options available to you. New drugs have been developed that are effective against drug-resistant strains of HIV. However, it is very important that these drugs are taken properly. If not, you may develop resistance to them, and this could mean that your HIV becomes very hard to treat.