What is the difference between / and ./ at the start of a file path in Linux?
/ means starting from `root` which is the top level directory in your Linux partition's filesystem.

./ means starting from the current directory.

You can see your current directory with the command `pwd` which is short for print working directory.
What does top level directory mean in this context? Can I run the command: `rm -rf /` in order to delete all of the files in the project that I'm working in?
/ is the top level directory of the entire system, not just for the current directory or user.

I must strongly recommend that you **do not** run the command that you asked about. `rm -rf /` would start from the top level directory of the system, and subject to user permissions, will attempt to delete every accessible file and directory on the entire system. 

To delete all of the files for the current project that you're working on, you should first change to the directory that contains that project. If you aren't sure which directory you are currently in, you can use the `pwd` command to check. Then you could use `rm -rf ./` to remove of all of the files and directories contained within the current directory. It is important to note that if multiple projects are being stored in a single directory then this command would delete all of them and would not be appropriate.