When you see the yellow status symbol for Skype contacts, you know their status is set to Away. This indicator may mean that the contact has been inactive for a certain period of time and may be away from the computer, or that the contact manually set the status to Away.
When you see the Away status, the contact may have been inactive for some period of time, though the exact amount of time varies because each Skype user can customize that duration. Someone whose status is Away can still receive instant messages, and calls to that person will connect, though that contact may not be available to take them.
To change your status to Away, click on the status icon by your name, and then click "Away" in the drop-down list that appears. You can also set Skype to automatically change your status to Away if you're inactive for a certain number of minutes. To customize this setting, click "Tools" and then "Options." In the General Settings portion of the General tab, check the option "Show me as Away when I've been inactive for," and in the text box type the number of minutes you prefer and click "Save."