When Llanowar Sentinel enters the battlefield, you may pay {1}{G}. If you do, search your library for a card named Llanowar Sentinel, put that card onto the battlefield, then shuffle.
Llanowar Sentinels can “chain”: If you put a second Llanowar Sentinel onto the battlefield as a result of this ability, that creature’s enters-the-battlefield ability triggers and allows you to search for yet another Llanowar Sentinel.
Because the “search” requires you to find a card with certain characteristics, you don’t have to find the card if you don’t want to.