
The button automatically updates based on the number of updates that will be installed. Notice that in this example there are eleven updates hence the button states 'Install 11 Items'. Click Install on the lower right corner.Select the items you want to install (uncheck the items you don't want to install).Some update will show a left pointing arrow, an indication that in order for the update to be installed your Mac, a restart will be required

Just click on an item to view its description in the lower pane. Software Update displays information about a particular update. If during the check updates are identified, a list will be created that shows the available updates. You'll see a a check for new software will initiate.

Pay close attention to the image in step five. See the following link that describes how Software Update works in Tiger.
MAC OS 10.6.8 SOFTWARE UPDATE MAC OS X
Instead of installing an update one by one and then showing you the status in Software Update (a behavior in Mac OS X Tiger), Software Update in Leopard will download all the updates you have selected and then installs them. For the most part, Software Update on Mac OS X Leopard works in much the same way as it does on Mac OS X Tiger.

MAC OS 10.6.8 SOFTWARE UPDATE FOR MAC OS X
If you want to manually check for Mac OS X and/or application updates for a Mac running Mac OS X Leopard, you can launch Software Update.
