RGBA works just like RGB, except that it adds a fourth value: alpha, the opacity level. The first three values represent red, green, and blue. For the alpha value, 1 means fully opaque, 0 is fully transparent, and 0.5 is 50% opaque. You can use any number between 0 and 1, inclusively.
background: rgba(0,0,0,0.2) url(../images/bg-form.png) no-repeat