The shared dream has a similar meaning for each of the men in Of Mice and Men, but is particularly poignant for George and may have twin meanings for him.
The first meaning of the dream for George is the most compelling and important one. He sees the possibility of owning a ranch as a change from his current position, as a permanent home, and as a way to finally take control of his own life.
For George, and the others, life consists mainly of work done for other people. George and Lennie travel often and have nothing to call their own and no place to call home. The dream represents the opposite of each of these conditions.
Another meaning of the dream that is particular to George has to do with his relationship to Lennie. George keeps Lennie entertained and happy by repeating the story of the dream to him. It is entirely possible that the dream was created for just for Lennie's entertainment and only later became a "real idea" (especially after Candy proposed to join the men and pitch in money).