A temperature inversion is a layer where the air gets warmer with height instead of cooler, as it normally does. This warm "lid" traps haze and moisture near the surface: visibility drops, fog brews, and it marks stable air.
In Latvia inversions are especially common on autumn and winter mornings. You can spot one without instruments: smoke from a chimney spreads out flat in a horizontal sheet instead of rising, and mist hangs over the fields.