Grunt and Gulp are both task runners.. they are utilities than can be configured to automate a set of actions on your project. Usually these are build tasks, preparing your code to be run. This can include steps such as copying files, minifying code, running transformations (such as compiling SASS/Less to CSS).
While Gulp/Grunt are still used, IMO webpack is the most important of the three to know. The learning curve can be a little steep, but it's very widely used.