An SR-22 is a financial responsibility form filed by your insurance company, which the department of motor vehicles requires some drivers to carry, in order to reinstate and keep their driver's licenses. This form states that you have at least the minimum required liability coverage in force. If for some reason, your policy lapses, the insurance company reports that lapse to the DMV and your driver's license may be suspended if you have not yet completed your required term.
*Tip: Each case is different so be sure to ask the DMV how long you need to carry the SR-22 for when you go to reinstate your license.