'Viral load' is the amount of HIV in your blood. It is likely that you will have this tested regularly as part of your care to manage the virus.
The result is measured in copies (of the virus) per millilitre (ml) of blood and it can range from over 1,000,000/ml to fewer than 50/ml - which is classed as 'undetectable', meaning blood
For somebody not on HIV treatment, a viral load above 500,000 is considered high, and one below 10,000 is considered low. But everyone's viral load is different and it is not unusual for one person's viral load to fluctuate quite widely.
It might temporarily increase if you're unwell with an infection, for example, and the time of day your blood sample is taken could also influence your viral load. So rather than attaching too much importance to a single test result, look at the trend in your viral load over time and discuss any concerns with your doctor.
If you have a high viral load then your HIV is more likely to be transmitted to someone else if you don't practice safe sex than it is if you have a low viral load. But having an undetectable viral load does not mean that you cannot transmit the virus. Undetectable means just that: it cannot be detected by tests, but the virus is still there.
If you are on medication then the viral load and CD4 tests will be carried out to decide how well the treatment is working and whether the dosage or the type of drugs need to be altered.