It indicates that the pressure is low in one or more tyres. You will not know how much until it is checked. You can buy a simple tester or and do it yourself or have it done.
I am not really impressed with these monitors. They seem to fail too often (generally false problem indicator. As a result many people ignore them.
If it were me I would get a good manual gauge and learn how to test it myself. Well I do test my tyres myself. It is quick and easy.
I don't suggest you ignore the possibility that the pressure may be low. Low tyre pressure can cause a real reduction in safety.