The default page is the webpage that appears each time you start your browser on the Internet. Internet Explorer usually sets it to a Microsoft site by default like Bing or Live. Quite often people will use Yahoo or Google for it or organizations will have it set to their web site. On my personal PC I have it set to my webpage.
To change it in Internet Explorer there are several options depending on your version.
In Internet Explorer 9 go to the page you want to use as default and then you will click the down arrow by the Home icon (looks like a house) and choose to change home page. Choose that option. Then choose whether you want to use it only as the default on the first tab or on the current tab or all tabs.
In the various versions of Internet explorer you can click Tools on the Command Bar (if it is not showing, right click in the area where bars are displayed an choose Command Bar) and then choose Internet Options and then type in the address of the page you want to use in the Home Page Box or if you are on the page you can just click Use Current. Then Click OK to close it.
If the menu Bar appears you can follow the steps in the last paragraph just choosing Tools in the Menu bar.