If your tickets are listed as GA, SRO, or Lawn, it means that there are no specific assigned seats in your section. You are able to stand (or sometimes sit) anywhere within that section. If you hope to be as close as possible to the front of these sections, it is important to arrive at the venue as early as possible.
Note: Sometimes you will see "seat numbers" listed on tickets that are also designated as GA. These numbers are for the venue's inventory management purposes only.