Shorthand for 'enters the battlefield,' the trigger condition on abilities that fire the moment a permanent comes into play, such as a creature that draws a card or deals damage when it enters. When an event puts one or more permanents onto the battlefield, all permanents there, including the newcomers, are checked for matching ETB abilities.
For new players
When a permanent enters, check every permanent on the battlefield for matching ETB abilities, including the new one. A permanent that was already there can trigger because of the arrival too.
You may use Solemn Simulacrum's ETB trigger to search your library for a basic land card, put it onto the battlefield tapped, and shuffle when it enters.