"BBW" stands for Big Beautiful Women. This term refers to women who are generally considered larger or curvier than the average, often due to their body size and/or bust size. In the context of porn video tags, "BBW" may be used to describe a video featuring a performer or performers with larger bodies and/or breasts.
In this case, "big-boobs" is an additional descriptor that emphasizes the focus on large breast sizes within the video. So, the tag "bbw-big-boobs" would refer to a porn video featuring one or more women who are Big Beautiful Women with notably larger than average breasts. This is often appreciated by viewers who have a preference for this body type and/or breast size.