Airmet means that there is a weather condition that could affect airplanes and pilots need to be aware of it.
Think of it like a warning for things that could make flying difficult or dangerous, like bad visibility, turbulence or strong winds.
When an airmet is issued, it basically tells pilots to be careful and maybe change their flight plan to avoid the bad weather.
Just like when you go outside and your mom tells you to wear a jacket because it's cold, the people who control the airplanes tell the pilots to be careful when there's an airmet.