The PATH variable is cruical for the command line and any program which runs commands in the backround (you will be surprised how many do!). The PATH is basically a list of directories where you can find programs to run on the command line. So rather than typing "C:\windows\system32\xcopy.exe" you can just type in "xcopy".
When you type a command in it will search every folder in the PATH variable to see if it can find it. So if you do not configure the PATH correctly then commands will not work. Why should you have a care face? Well if you do not ensure java and javac commands are on the command line then you will not be able to run or compile Java!
You can test to see if the PATH is correct by typing in
Java -version
if you get "command not found" then cry! But do not cry too much as we can fix it! Find where the "java" and "javac" commands are on your computer. You can do a search for it if you want. They will be in a folder called "bin" inside where you installed java in. Copy the path (including the bin). Then type in the following -
Windows - SET PATH=%PATH%;C:\java\bin;
linux - export PATH=$PATHl;/usr/java/bin;
Note that you should replace "C:\java\bin" with where you found the javac program. If you could not find the javac program then reinstall the Java JDK!