The moisture meters are devices used to measure the percentage of water in some place or substance. This information is used to determine if something’s ready to go, if it is too wet or too dry, or if some material needs to care more.
At home we use several types of moisture meters, some to the gardens and others to measure how wet is the inside of the house or the woods we have. Or maybe the basement, if you have one. But that’s not something you get used much on this side of America, so I would rather be in gardens and sometimes walls.
The first instrument I mention they are tensiometers and are referred to doctors, also measure the amount of water on earth, but is used in places where the landscape is more professional. The gardeners could use one of these and inform you how much water they use to water the plants, especially if your form is drip irrigation.
Then another is easy and simple hydrometer, device that is placed on the ground when you want to see how much moisture is in a zone. They are the best to avoid excess water, and makes the plants do not feel stifled. These devices are readily available and cheaper, but only tell you if the soil is dry or damp, therefore, the amount of water that you apply to your plants will depend on what it is. For example, a cactus needs almost no water and is usually dry land.
To measure the humidity inside the house there are other tensiometers, which will tell you how much there is and also the temperature. And just stick to the wall. There are other more professional, using services that measure moisture in wood and basements, but are difficult to use, specific and very expensive models.