What does Mark 2:23 mean?

Sabbath" or Shabbat is from the Hebrew root word shin-beit-tav meaning "to cease, end, or rest." The Sabbath is an extremely important part of Judaism. It is the fourth of the Ten Commandments and God commonly condemned the Israelites when they refuse to honor the Sabbath (Ezekiel 20:13, 16, 21). The purpose of the Sabbath is to honor the seventh day of creation, on which God rested (Exodus 20:11), trust Him for provision (Exodus 16:22–26), rest (Exodus 20:9–10), exhibit righteousness (Exodus 31:14), and especially to display a distinct separation between Israel and pagan nations (Ezekiel 20:12).

Per customs and laws of the time, the disciples are not stealing by plucking grain heads out of someone else's field. As long as they use their hands and not a sickle, it is allowed under Deuteronomy 23:25. The question at hand is if plucking grain is allowed on the Sabbath or does it fall under the category of "work" as given by Exodus 20:10?

Around the time these events took place, Jewish leaders had created thirty-nine Melachot (working) prohibitions to ensure no good Jew came close to breaking the Law. It's reasonable to assume these Pharisees lived by similar prohibitions. By plucking grain, separating the wheat from the chaff, disposing of the chaff, and eating the grain, the disciples violate at least four of the Melachot: reaping, threshing, winnowing, selecting.

It's important to note that Jesus neither plucks nor eats the grain. His disciples do, but as their Rabbi He accepts responsibility for their actions.