It is a flowering bush with many tiny white blooms that bloom early spring. My grandmother always had these flowering bushes on her farm near the farmhouse in Alabama. They tend to like full sun. Her nickname for them used to be White Shirt Buttons.
this is a favorite early Spring flowering shrub called the Bridal wreath spiraea, Spiraea prunifolia ‘Floreplena’. The ‘Floreplena’ means that the flowers are doubled. It usually flowers before the leaves come out, and I’d call it an old-fashioned popular shrub. There is a good discussion of it here: http://www.floridata.com/ref/S/spir_pru.cfm I like your grandmother’s nickname for the plant! It is native, by the way, to China.
Steve Hill, Botanist, SCNPS