What are proprietary drivers?
Most of the devices (hardware) attached to your computer should function properly in Ubuntu. These devices are likely to have open source drivers, which means that the drivers can be modified by the Ubuntu developers and problems with them can be fixed.
Some hardware does not have open source drivers, usually because the hardware manufacturer has not released details of their hardware which would make it possible to create such a driver. These devices may have limited functionality or may not work at all.
If a proprietary driver is available for a certain device, you can install it in order to allow your device to function properly, or to add new features. For example, installing a proprietary driver for certain graphics cards may allow you to use more advanced visual effects.
Many computers do not need proprietary drivers at all because the open source drivers fully support the hardware.
Most problems with proprietary drivers cannot be fixed by Ubuntu developers.