What does Mark 2:24 mean?

Jesus' interactions with the Pharisees have finally escalated to the point where they question Him directly. The word translated "it is lawful" is from the Greek root word exesti. Exesti is comprised of ek, "originating from," and eimi, "to be." So, literally, the Pharisees are saying the disciples' actions do not come from a place that should be. The act of plucking grains on the Sabbath, according to their view, "shouldn't be."

In an attempt to ensure the people come nowhere near breaking the Sabbath, the Jewish leaders will enforce thirty-nine clarifying rules, or Melachot, that specify what is and is not allowed. In this case, their definition of "work" originates from six actions that needed to be accomplished to build the tabernacle: making the showbread, making the fabric curtains, making the leather curtains, making the beams, erecting the structure, and finishing the final touches. Of the eleven prohibited tasks related to making the showbread, the Pharisees considered the disciples guilty of four: reaping, threshing, winnowing, and selecting.

It's likely the Pharisees accepted these Melachot in their oral law before they were written in the second century AD, but are the disciples guilty of breaking the Mosaic Law? Did plucking grain heads and removing the husk qualify as work (Exodus 20:8–11) or preparing food (Exodus 12:16)? Leviticus 25:1–7 prohibits working in a field during a Sabbath year, but makes no mention of a Sabbath day.

Jesus doesn't argue with the Pharisees about the finer points of the Law. Instead, He addresses their hearts and the hypocrisy they live under.