You can buy beer to have fun and not seeing it as a waste of money. You can give a friend little amount of money in a way he may want to give you back but you will not collect it as you see it as small amount, telling him it is a small amount of money. Only such money like that, that are small in amount set aside should only be used for gambling. That is the meaning. The fun you see there does not necessary mean fun. Just like an ideom.
This is a good way to explain it but also gambling for fun means really having fun while gambling and not doing it only as you want to make money. You can gamble for money at time and also gamble for fun other times. It doesn't mean that everytime that you're gambling that it has to be for fun or to make money. There are times when I need a feel good feeling which comes from dopamine that we get when gambling so I gamble and have a feel good time without thinking about losses.
Gambling for fun will not make you addicted and this is why gambling for fun is better than gambling to try to make money. If you're looking for money, there are better ways to get them and you can get the money without depending on your luck. Money needs working therefore if you get a job either a part time or a full time job, you can get salary and be living comfortably without depending on gambling for money.