Water filters are very useful in just about every survival or disaster situation. Drinking the water directly from a water source could put you in danger of getting sick or even worse. There are many contaminants that can flow freely in water so it's important to filter them out.
When you use a portable water filter, Water passes through, but bacteria and protozoa do not.
A portable water filter will have two tubes. You will use one tube to stick into the water source, and the other tube will go into your container. Next, you will pump the water through the filter and into the container.
They are lightweight and easy to put into your pack. Some portable filters will actually screw onto the top of your water bottle. Pumping the water can take some time. The average filter will produce 3 liters of water in 10 to 15 minutes.
Having said that, clean water is essential to survival, so taking the time is so worth it. If you are hiking and you find a good water source, don't pass it by without filling up your water containers. Otherwise you risk being far from water when you need it most.
Here is a short video where Jeff demonstrates a water filter in action:
When you buy a filter, make sure you test it out before you get into a survival situation so that you are familiar with it and how it works.
Most of all, enjoy the clean water!