Server programs, when addressed via the network, need to be allocated to a certain network port. This way, multiple diverse server programs may run on one machine. Every service has a default port, which initializes the program. For example, a web server is initialized on port 80, the MySQL database server utilizes port 3306.
MAMP allows you to determine which ports the server should respond to for incoming connections. MAMP does not utilize the standard ports in its factory setting. That way, the MAMP servers can run additional to standard web and MySQL servers already installed on your Mac OS X. Should ports 8888 and 8889 be in use by a different application, please change the values accordingly.
The button "Set to default Apache and MySQL ports" will set the ports to the value commonly used on the internet. This can be useful, if you want to access the web server of your MAMP installation with DynDNS from the outside. The button "Reset MAMP ports" will reset the ports for Apache and MySQL to 8888 and 8889.