The European view of land, especially at the time when Europeans first came to Australia, was that land belonged to those who occupied and developed it. There was a doctrine called terra nullius that basically said that any land that was not being lived on or developed by anyone was unowned and could be taken. To Europeans, then, land was something to be parceled out, developed (with farms, buildings, etc) and owned.
To the Aboriginal Australians, land had a more spiritual significance. The land was not something to be owned so much as something to be lived with. It was, in a sense, part of their culture and part of themselves. This was a much more spiritual connection to the land, one that was different from the simple idea that people own and control the land.
An important problem was that the Aboriginals did not use the land in ways that the Europeans could recognize. They moved from place to place instead of settling down in one are. Because the Europeans could not see that the land was being (in their eyes) owned and used, they took it for themselves.