The Default text entry field lets you choose an initial string of text to be used by the parameter when the plug-in is first applied. Contrary to the simpler Text Field parameter, the Multi-line text parameter accepts text which spans multiple lines.
The Multi-line Text parameter feeds a single string value to the Quartz composition published input it is mapped to. Each line can be extracted by using the String Components patch and entering the Return character as the separator (type Option-Return).