What does Luke 10:26 mean?

A lawyer has asked Jesus how he can "inherit eternal life" (Luke 10:25). Jesus in turn asks the lawyer about the Mosaic law; He likely does so for a couple of reasons. First, Jesus proves He is also a devout Jew who honors the Mosaic law. He's not trying to turn Judaism into anything God didn't intend.

Second, this lawyer is a Pharisee, as shown by his question about eternal life, which the Sadducees did not believe in. While Sadducees followed only the written Mosaic law—the Pentateuch—Pharisees included the "Oral Law," which is extra regulations they claimed God gave to Moses to supplement the written law. The Pharisees try to force the Jews to follow the Oral Law as well. Jesus says, "They tie up heavy burdens, hard to bear, and lay them on people's shoulders, but they themselves are not willing to move them with their finger" (Matthew 23:4).

The Oral Law wasn't written down until about AD 200 when the Jewish teachers wanted a standard for the Jews in the Diaspora. By asking what is written in the Law and how does the lawyer read it, Jesus is turning the lawyer back to the written Mosaic law and away from any extra-scriptural teachings (1 Corinthians 4:6).