https://github.com/leoliu/ggtags is an Emacs package for using GNU global. After indexing your project, you can use this package to jump to the definition or references of a tag (for instance, a variable name). IDEs have similar features, and this is one way to get “find usages” or “find definition” functionality in Emacs.
I don’t know how it compares to other methods (eg. ctags and the like) but from the README it appears to be more advanced, and it’s more actively maintained.