What does an exclamation mark mean in front of an Eclipse import project?

1. In particular, the project is copied from other places and loaded directly, just open will often see the project icon has a red exclamation mark, because the build path error, there are missing or unable to find the package.

2.

Cause: The red exclamation mark is displayed because the path to the jar package is incorrect

FIX: On project right click on Build Path---Configure build paht ... (or Propertise->java Build path), libraries in Javabuild path, the package with the Red Cross is the wrong path.

After processing, return to the Refresh project can, this problem is generally imported into someone else's project or before some jar package moved its storage location.

PS: Project Yellow Shield exclamation point, indicating that there is a reference to the jar package, but actually do not use the jar package (can be understood as defined variables, but used in the future)

What does an exclamation mark mean in front of an Eclipse import project?