Viral load is how much HIV is in your body fluids. The higher someone's viral load, the more HIV is in their body fluids, meaning a greater risk that the virus will be passed on during unprotected sex.
HIV treatment usually brings your viral load down, often so low that it is classed as 'undetectable'. This doesn't mean that all HIV has gone - it is still there but in amounts too small to detect.
There is a lot of discussion about how likely it is for someone who has had undetectable viral load for at least six months to pass on HIV. Some experts in Switzerland say HIV will not be passed on within a couple if they are monogamous and free of sexually transmitted infections; but there are cases where people with undetectable viral load have infected others.
Before you and your partner make any decision about not using condoms, get advice from an HIV specialist.