Even though we take batteries, and in particular Lithium Polymer batteries, for granted is that we use them every day in mobile phones, cameras, toys, electric tools, my toothbrush, even cars. They "just work" and seem very safe. All of these are safely inside a device or in some proper hard case to use with some dedicated (or built in) charger which includes software and hardware to manage the battery and make sure it stays safe.
However, in the model and radio control hobbyist world, people often use much more "raw" LiPo batteries, which are simply wrapped in a foil or soft plastic. They are charged with simpler circuits and need a lot more care and attention.
In practice these devices carry a lot of energy, and that makes them dangerous. These "raw" batteries are much more so as it is very easy to charge or discharge them incorrectly and very easy to piece the wrapping or damage them. When damaged they can catch fire, and release that energy. The fire is hard to put out, but usually not long lasting. It can very easily cause secondary fires.
The battery safety aspect of the course covered several areas, and had some nice scary videos of LiPo fires. I did however find some of the points made a tad odd, and I checked the instructions that come with the DJI batteries, and they are interesting. The Phantom 3 batteries have instructions covering things we had on the course, like store with 30% to 50% charge. Though the Phantom 4 batteries have instructions saying 40-65% charge for storage.
On the course they also said these batteries self discharge, and will discharge to an unusable state where the battery cannot be charged. The instructions (Phantom 4) say they discharge to 65% automatically over 3 days and then will hibernate, and can be charged after they have hibernated to get them back.
Comparing the two battery instructions shows that they have improved the batteries a lot from Phantom 3 to Phantom 4, and that may be why the details on the course are somewhat confusing and perhaps simply out of date. Ultimately the course should say READ AND FOLLOW THE INSTRUCTIONS. And especially do not assume the instructions are always the same.
That said, the Phantom 4 instructions do have a troubling couple of points. They say, sensibly, "Make sure the batteries are fully charged before each flight", but also "Never transport a damaged battery or a battery with power level higher than 50%". Err, and if you are going to a field with no power, how do you manage - you have to transport fully charged batteries.
Another odd one for such an intelligent battery system is "DO NOT charge battery immediately after ight, because the battery temperature may be too high". Sensible, as I bet people do that all the time, except why the hell does the intelligent battery system not know it is too hot and simply not start charging until temperature is OK, that way you can "put on charge" right away.
There are good tips on ensuring battery contacts are taped, and better still, batteries are carried in fire-proof LiPo bags or ammo containers.
One thing to always remember with any battery, even in your phone, is that you must not charge or use if the battery is damaged. That is how fires start, and is very dangerous. A damaged battery is unsafe. When you are flying a small aircraft like the Phantom 4, you have to be especially careful if you do have a crash as that can easily cause damage to the battery - making it unsafe, or even starting a fire immediately.
So, take battery safety seriously!