An SR-22 is a certificate that an insurer files with the driver's state of residence to certify that the driver is insured. "SR-22 insurance" may refer to a requirement in some states that a driver must file and carry the SR-22 after certain violations.
An SR-22 is only required if a judge has ordered it. The exact requirements vary by state, but they usually involve serious violations or a string of repeat violations. There is a nominal added fee to file an SR-22 with the state. Certain states also require that drivers ordered to file an SR-22 must also pay their insurance in full up front.