Remember one thing that selenium IDE software automation testing tool can handle only one window at a time. When you are taking some action like clicking on link or button and open new popup window then how to handle it in selenium IDE software automation tool? "selectWindow" command will help you to select window but before that you need to verify that your expected window is open or not. "waitForPopUp" command will help you to wait until your expected window is not open.
In bellow given example, "waitForPopUp" command will wait for the popup window for 30 seconds.
|selectWindow||title=FeedBurner Email Subscription|
|assertTitle||FeedBurner Email Subscription|
|verifyTextPresent||Thank you for your request|
Note : You can Read complete tutorial on selenium IDE commands with examples at this link.
In above given example, "selectWindow" command will find window with title=FeedBurner Email Subscription and select that window to perform all next actions.
If you will give target as a null then again selenium IDE software tool will select main window. "selectPopUp" Command is very similar to "selectWindow" command.