「一生懸命」 (いっしょうけんめい) means "with utmost effort" or "diligently." You use it to describe doing something very seriously, putting all your energy into it. For example, when studying for a test or working hard to achieve a goal. He is studying Japanese with utmost effort.

「一生懸命」は、できる限りの力を尽くして物事を行う様子を表します。彼は一生懸命日本語を勉強しています。