Structure > Channels > Channel > Main > Hide From Search Results

The "Hide From Search Results" option on a Channel serves to not index any of the content that is under the channel.

This means that, if you search for an article (i.e. on an Article or Event listing) from that channel and the channel has "Hide From Search Results" enabled, then the article wont appear.

In other words, the "Hide From Search Results" option prevents the content of the Channel from being indexed by the Cloud Search, which is used by Site Search, Scoped Search and Forum Search. If you enable or disable the option, a re-indexing of the search results will be triggered, which may take up to 24 hours depending on the amount of content in under the Channel.

There also other reasons why an article would not appear, such as the Publish End date arriving or the Live option not being ticked.