The unix shell provides a command-line interpreter. The shell provides an interface between the user and the kernel. The shell will pass your commands to the operating system. Mastering basic shell commands will help you speed up and automate a variety of tasks.
You can get started with unix by working through the exercises in The Unix Shell.
More advanced material is available in Extra Unix Shell Material.
A linux commmand line tutorial focused on ubuntu is available at The Linux command line for beginners.