Valve extenders are useful for inflating your tires when a bike pump can't easily attach to the tire valve. With 10" tires, valve extenders may not be necessary, but are certainly helpful. However, valve extenders are usually needed for 8.5" tires as the valve sits deep within the rim.
The valve extender has threads that screw onto your valve. Once you have removed your valve cap, the extender will screw on by turning to the right. After that, just attach your bike pump tightly to the valve extender and you're ready to start pumping!
For visual instructions, please check out this video on using the valve extender: