The GPL stands for the "GNU General Public License", and is a commonly used open-source license.
There are other open-source licenses that are similar to it.
You may have heard about its "viral" nature, that forces code that uses GPLed code to be open-source as well. Should you worry about it?
Usually there's nothing to be fear. The GPL (and all other free software licenses) explicitly allow making use (and even modifying) the software for internal use.
Software that operates web-sites is considered software for internal use, as it is not distributed to the outside.
Amazon.com as an Example
If we take Amazon.com for example, then they may make use of GPLed code for their web-site.
If, however, they decide to distribute the code as a framework that allows setting up similar sites (say Amazonware), then they'll have to either comply by the terms of the GPL, and distribute it under a compatible free software license, or alternatively eliminate the use of the GPLed software.
(Or if the option permits, get the copyright owner of the GPLed software to exempt them from the GPL somehow.)
Conclusion
This presentation was directed primarily at people who manage or wish to set up web-sites.
If you want to sell a framework that will facilitate setting up web-sites using LAMP, I wish you the best of luck, but you'll have to handle the legal problems involved in making use of open-source (and proprietary) software available on your own.
Generally note that the distribution terms of proprietary software may give you even more trouble, as far as basing a web-site on them is concerned.