The number after the dash on a S&W, in this case an 8, is called an engineering change. When S&W makes significant internal or external changes to a model, they denote this with a new "dash number". Dash numbers can also represent something such as a different barrel length, with everything else about the gun remaining unchanged. For example, a 36 no dash and a 36-1 are the same gun, except the 36-1 has a 3" barrel instead of the 2" barrel on the no dash. An engineering change can also denote the deletion of features, such as a pinned barrel or recessed cylinders.
For your specific gun, the -8 engineering change represents the addition of the internal lock for .45 ACP Model 625s.
