A mask fit test is a way to know how well a mask works for the person wearing it. You don't need to do a fit test to benefit from wearing an N95-style mask and most people wearing an N95 can achieve around 90% effective filtration without a fit test.
A fit test gives you confidence that you are probably getting less than 1% inward leakage when wearing your mask. This means that rather than getting around 90% protection, you're probably getting around 99% - a 10x improvement in protection!
In order to get a good mask fit your face needs to be clean, razor-shaven (if you have facial hair). If you cannot razor shave your facial hair, a large elastic elastic rubber band may be used to achieve a good fit.
Make sure your mask intuitively fits you well and you don't feel jets of air leaking out of it when wearing it. This kind of test is usually called a "seal check." The video above demonstrates how to do this kind of test. If you pass this "seal check" step you are probably getting at least 90% protection.
If interested, please see the technical details on this method of fit testing.