AutoLISP is a programming language that works with the full version of AutoCAD. Though it doesn't work natively with AutoCAD LT, there are 3rd party addons that do enable AutoLISP for AutoCAD LT. At it's most simple level, AutoLISP can be used to automate repetitive tasks to help increase your AutoCAD productivity. For example an AutoLISP program that I use all the time can calculate the total area of all polylines on each layer in the drawing - HUGE time saver. At the other end of the spectrum AutoLISP can create hugely complex programs that can vastly increase the functionality of AutoCAD.
Drag the AutoLISP file onto your AutoCAD drawing. This is very quick method. The downside is that the AutoLISP is only loaded into the current drawing session and is not retained when the drawing is closed. Hence will need to be re-loaded every time the drawing is opened.
Using the appload command and loading the AutoLISP file into the startup suite. Then every time you open a new drawing the particular AutoLISP file will automatically be loaded.
This method is aimed at advanced users and CAD managers. I won't go into any detail but just mention it so you're aware of it. You can amend the acaddoc.lsp file to load the particular AutoLISP file automatically whenever you open a new drawing.
I'm happy to admit that I'm a bit of a novice with LISP. I know the basics and can create some simple programs to improve my productivity. If you want to know more there's lots of resources online. Try AfraLISP and Lee Mac for starters.