Adding a compiled application to $PATH. Understanding the problem
The PATH variable already containts a list of directories which the shell (which is what is running inside Terminal) is using to search for any executables. So if you compile something on your own and put it into one of the folders listed in PATH you can execute it from the shell prompt right away. 

The current folder (.) usually is not part of PATH for security reasons. So for stuff you compile and want to run right away, you have to execute ./stuff for the shell to find it.