A “light novel” usually refers to Japanese novels, typically written for YA audiences, which are supplemented by occasional illustrations. Typically, you have manga (all illustration - like a comic book), light novels (mostly words, but some illustrations), and novels (no illustrations, all text).
Many popular light novel series have been adapted in animation. Examples include: Sword Art Online, Log Horizon, A Certain Magical Index, Irregular at Magic High School, Shakugan no Shana, All You Need is Kill (adapted as a film in the US - “Edge of Tomorrow”) and Kaze no Stigma.
If you’re interested, a number of them have been translated by fans and posted online at various places. A google search will probably help you find them.
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What are light novels?