It simply means there is missing footage. You may want to read the online help on such basics. If there were additional assets with the template you may have to download them, otherwise these could simply be the images you are supposed to replace with your own. Follow the instructions of the template.
As Mylenium, there's missing footage. If you use the search box in the Project panel to search for the work "missing" After Effects will highlight any the missing files.
If you then double-click on the file, your Windows Explorer or Finder(?) will open and let you locate a file with the same file name. Once selected, any other missing files which are in the same folder will automatically be updated too.
If you don't have the file, you can right-click on it in the project panel, you can then choose to Replace footage