Can anyone help me understand this? — Mojtaba

Kant is illustrating his categorical imperative by applying it to an example. It's also a reductio ad absurdum which works in his favour.

Note how he takes something individualistic and turns it into something universal to see how it would pan out? That's the categorical imperative in a nut shell.

A more readily understandable example along the same lines might be if you found a wallet full of money lying on the ground, thought that you could get away with picking it up and keeping it for yourself, and you were motivated to do so because you want to get your hands on as much money as possible - in fact, you made that your goal in life.

Now consider, what if everyone thought like that?

"I have made it a personal rule to make make myself richer in any way I can, so long as it's safe. Somebody gave me cash for safe-keeping. Then they died. There is no paperwork. YaaY! I get to keep the money!

"But wait a minute! When I decided to make myself richer, I supposed that my "law" could not hurt myself. But suppose everyone adopted my "law," If everyone did that then where would I be? My "law," if I apply it, would ultimately hurt me!

"It looks like any "law" that is similarly self-destructive is just plain untenable, even crazy, and cetainly indefensible and of course irrational."

And there you have it, an example of how Kant's categorical imperative might work. The idea being that if you make rules for yourself, you ought to see if they could at the same time be the same rule for everyone else, the test of reason. If not, then it's not a good rule. Kant uses the word "maxim" because the "rule" he's concerned with is general in form. For example: I want to convince myself it's OK to rob Smith - .I'm sure I will not get caught. But the general form of "it's OK to rob Smith" is just "It's OK for anyone to rob anyone(!)" Well, it's certainly not OK if someone robs me, therefore, it's not OK for anyone to rob anyone, and therefore, not OK for me to rob Smith.

Kant is not an easy read. Hang in there and you will find that just as your muscles get used to new tasks, getting stronger in the process, so Kant will become easier to understand, and your general reading skills will also benefit!