what is only screen in media query code example

Example 1: media query

@media (max-height : 800px){ /* from 0 to 800 px max height applies */
  p{
    font-size:10px
  }
}

Example 2: media query only screen

@media not|only mediatype and (mediafeature and|or|not mediafeature) {
  CSS-Code;
}

meaning of the not, only and and keywords:

not: The not keyword inverts the meaning of an entire media query.

only: The only keyword prevents older browsers that do not support media queries with media features from applying the specified styles. It has no effect on modern browsers.

and: The and keyword combines a media feature with a media type or other media features.

They are all optional. However, if you use not or only, you must also specify a media type.