Normal humidity within a home should typically stay between 35 and 45 percent, depending on local temperature and climate. Various methods exist that help to either bring it down or up as needed.
A common way to measure humidity is with a device called a "hygrometer" - this small instrument shows what the relative humidity level is in an area.
When humidity is too high, damage can occur to various personal belongings, such as musical instruments and wooden furniture, and to aspects of the home itself, such as the drywall. High humidity can also allow for the breeding of bacteria and the build up of mildew.