Enabling a block simply means to "turn it on". The way to turn on a block in Drupal is to place it in a region via the Blocks manager.
Note, if you will be configuring your block, it's best to do your configurations first, save them, and then enable the block.
To enable a block, just do the following:
Go to the Blocks manager at Admin menu > Site building > Blocks.
Locate the block you wish to enable in the Disabled list towards the bottom of the screen. In the corresponding drop-down menu, select the region in which you would like to place the block. Once you select the region, it seems as if the block "disappears", but it's not gone - it just moves itself to the region you directed to move to when you made the selection.
If there are multiple blocks in the region and you want to adjust the order of the block in relation to the others, just grab the directional "handle" of your block and drag it up or down accordingly.
Click the Save blocks button at the bottom of the screen.