And Jesus withdrew to the lakea with his disciples, and a large crowd from Galilee followed him. And people from Judea, Bible see other translations
This is referring to the “Sea of Galilee.”

“the lake.” The “sea” of Galilee is actually quite a small lake, only 7 miles (11.2 km) across and 12 miles (19.3 km) long, and the entire lake can be seen from the escarpments on both the east and west sides. The Greek word thalasso, lake, sea, or ocean, does not really refer to the size of the body of water, and thus has to be translated into the English “lake,” “sea,” or “ocean” by knowing the body of water that is being referred to (see commentary on Matt. 4:18).

“a large crowd from Galilee.” Mark 3:7-8 seems to actually describe two different sets of people, one from Galilee where Jesus had his headquarters and spent most of his time, and one from many other places where either Jesus had visited or word about him had reached.

“followed him.” Jesus’ headquarters was in Galilee, so he was near the people of Galilee most of the time, so they could “follow him.” In contrast, the crowd that had come from further away had to travel to get to where he was, so the text says those people “came to him” (Mark 3:8).

“And people from Judea.” The idea of “people” comes from the third person plural verb “came” (“they came”) at the end of the verse.

Commentary for: Mark 3:7