The soundcard driver is a program that mediates between Windows and your soundcard hardware. ASIO soundcard drivers are specially designed for music applications and give lower a CPU load and more responsive playback. They are also a prerequisite for audio recording.
Not all soundcards ship with native (their own) ASIO drivers so FL Studio installs with the universal ASIO4ALL soundcard driver. This video is for those people who are planning to use the ASIO4ALL soundcard driver with FL Studio.
The manual has a full section on troubleshooting problems with ASIO4ALL if this video doesn't cover some problem you are having. If that doesn't help then registered FL Studio users are welcome to ask for technical support and our team will be happy to help you out.
BTW: If you follow all the steps in this video you will be able to use multiple input and output devices simultaneously in FL Studio. For example, a USB mic (INout) and soundcard (OUTput).
Enjoy your ASIO4ALL