Receiving attachments via email is a bit of a two edged sword. On the one hand it is quite an easy way to get stuff, but on the other hand you don't necessarily know if the attachment contains wondrous information which will transform your life or viruses and other malware which will destroy your computer.
In order to help users, when they double clicked on an attachment Lotus Notes would bring up a dialog box warning the user of the possible perils and offering some alternatives including "viewing" the attachment to check that it was safe before launching it. But these days most computers and particularly those ones that are used for business have Anti Virus software installed to watch your back. In which case if you add AttachmentActionDefault=1 to your notes.ini file then double clicking an attachment will launch it in the native application with no dialog box.
More details are here: http://www-01.ibm.com/support/docview.wss?uid=swg21292126