What provision of the homestead act encouraged people to move west of the mississippi river?

It gave people free land - 160 acres - west of the Mississippi,as long as they went and lived on it for at least 5 years and showed clear signs of having improved (ie cleared and cultivated) said land. 

The land available included good farming land near to rivers,so was an attractive proposition for families that were willing to move westwards and set up homesteads on the land given them.

This link explains it