Don't worry if you didn't really get all of that. In every programming language we want to be able to change the flow of the program based on various conditions. For example if a file is in this directory do one thing. If it isn't do something else. So if the expression is true the statements inside the if block are executed.
Lets look at a simple example of the if statement.
You should try to comment you scripts as much as possible because someone else may need to look at it later. In six months you may not remember what you were doing, so you might need the comments as well. Using the if statement we can now correct some of the errors which would occur in the lowercasing script. In LCem.sh the script will hang if the user doesn't input a directory as an argument. So lets add this new knowledge to our program.