edit: phoog beat me to the short answer, but offers less background.
If you look up the 1 Train at the MTA website, you’ll see that it does not stop at Cortlandt St as that station is temporarily closed.
After the September 11 attacks, a number of subway stations were closed. Their treatment in subsequent maps is presented clearly on Brian Abbott’s blog:
Most service was restored by October 2002, but the Cortlandt St stop remains closed, and so remains on the map as grayed out pending reconstruction of the World Trade Center area.
—
The station finally re-opened as WTC Cortlandt St. in 2018, with 1 Train service, and is accessible via an underpass from the WTC Transportation Hub.
It means the station is closed. In this case, the station was closed after it was destroyed on September 11, 2001, and it has not yet been reopened. I believe it will be reopened sometime in the next few years as construction is completed on the site.
Update: The station reopened on September 10, 2018.
Credit:stackoverflow.com‘