What does the name Reuben mean?

It means "Look, I have a son." He was Leah's first son in the Bible, so she named with as almost a sense of astonishment as well as a celebration that she was the first of Jacob's wives to bear a child.

"behold, a son"

Reuben means "behold, a son" in Hebrew. In The Bible, he was the oldest son of Jacob and Leah.