RGBA extends RGB with an alpha channel from 0 (fully transparent) to 1 (fully opaque). `rgba(0, 0, 0, 0.5)` is black at 50% opacity.
Modern CSS also accepts space-separated syntax and percentage alpha: `rgb(0 0 0 / 50%)`. Both work; the older comma form is more widely understood.
If you only need partial opacity on a solid background, RGBA is usually cleaner than layering opacity on the whole element (which would also fade its children).