When the internet was first created it was not expected to be a place that you would have to worry about items such as security.
It was thought that the openness of the web would lead to people being able to learn from each other and so far that has been true.
This is why on any browser, if you want to see how a web page is built, all you have to do is hit the view source button and you will be able to see the source code for yourself.
But this openness has lead to security problems too.
On a normal program or on web sites that has most of their business logic on the server portion of the site it takes an attacker a long time to be able to find an exploit.
They either have to reverse engineer the program or run a lot of tests to see where the weak spots are.
The source code is right on the page so any skilled person can just take that and read it.
None of these techniques are fool proof but at least it will make it a lot harder for someone to be able to exploit your web page.
This will help fill a lot of the holes.
Again, this is not a surefire method for security but it is able to keep most of the bad guys from being able to exploit your code.
You do not want it to be the reason why your web site is hacked.