What does the following yellow icon mean in Eclipse?

When you use a version control system, such as Subversion or Git, the project is saved in a repository (server), usually different from the computer where the project itself is developed.

When you connect to that repository and download a project to modify it (in this case using the Eclipse IDE), a local copy of its files (usually all of them) is created on your computer.

You can then make changes to your working copy of the project, and later, when everything is correct, push your changes to the repository.

This allows collaborative work in parallel, since two programmers can modify different parts of their local copies of the project on different computers, and later upload their changes to the repository and synchronize with it.

Now, the symbol you mention (the yellow cylinder) appears when the local copy of a file is in the same version as the repository file, that is, the latest version of that repository file has been downloaded and you have not made changes in the local copy, so it is said that the file "is synchronized" or that it is in its latest version of the repository.

As a final note, in eclipse, when you modify a file in your working copy of the project, the yellow cylinder changes to an asterisk, indicating that the changes to that file have not yet been pushed to the repository.

Another case is when you create a new file in the project locally, that is, it does not yet exist in the repository. In that case, a blue question mark appears, indicating that it is a file that only exists on your machine, so it will have to be uploaded to the repository so that the rest of the team has access to it.