Many online journals and databases require subscriptions (which NYU Libraries pays for) to get access.
If you are on campus, access can be relatively seamless since authentication is usually established by IP address (e.g. if you have an NYU IP address, the publisher recognizes it and lets you in).
If you are off campus, then you need to be routed through the Libraries' proxy server, so that you can be authenticated after providing your NYU NetID and password. This page describes the main ways to do this.
Note: The proxy prefix and bookmarklet will only allow access to fulltext if NYU Libraries has a subscription to the resource. To see what online resources NYU Libraries has access to, use the "Books and More" catalog search box in the center of the library homepage.