MEL - Maya Embedded Language - is one of the scripting languages in Maya. It's a scripting language at the heart of Maya. Maya's user interface is created using MEL, and MEL provides an easy way to extend the functionality of Maya. Everything you can do using Maya's graphical interface can be automated and extended using MEL. Familiarity with MEL can deepen your understanding of and expertise with Maya.
Writing MEL scripts
Please refer to the page: Writing MEL scripts.
You can both create interfaces using MEL scripting as modify the existing Maya interface. Adding an interface to your script can greatly improve the usefulness. Being able to modify the Maya interface can improve the efficiency and work flow for advanced users.
Please refer to the page: MEL Interfaces.
Writing simple tools
Once you get the hang of the basic MEL scripting stuff, you'll probably want to start writing some useful tools. Some pointers to get you going and an aid to overcome some unavoidable 'bumps' you'd encounter when writing your first scripts, can be found on the page Writing simple tools.
Scripting involves quite some troubleshooting, especially at first. MEL Troubleshooting helps you with that, as it may be quite daunting at first.