Essentially, what this action by Congress said was that blacks did not really matter, that they were not as important as whites, even whites who had rebelled against their country.
Blacks had hoped that the lands confiscated under the Confiscation Act of 1862 would be used to give them a start towards becoming free farmers. However, Congress chose to return that land to the heirs of the former owners. This shows that Congress (and even President Lincoln) did not care enough about the slaves to give them a way to get a start in life. Instead, the government showed more concern for the white landowners (even though they had rebelled) than for the ex-slaves.