In this example, we'll be showing you how to filter your GA page views to only show iOS pageviews 

1.  log into GA and select the appropriate account and profile
2.  In the left nav, select "Site Content" then "All Pages"
3.  Under the graph, select the secondary dimension for your data set.  In this case, we want the secondary dimension to be "Operating System" (Under Technology)

 sec-dim.png     

4.  You will see an additional column has been added to your result data set.
5.  to only include iOS in your result set, click "advanced"
6.  Then enter iOS in the text box (see screenshot below), then click apply.
7.  Your results are now filtered to only show iOS page views

sec-dim.png