Forced anal is essentially a sexual role playing activity between a dominant and a submissive with anal sex at its center. In any forced activity scene, the premise is that the dominant partner is forcing the submissive partner to partake in the activity; in this case, anal sex. This may entail restraining the submissive partner so they are at the dominant partner's mercy.
Forced anal is also a very common scenario in pornography. Although these scenes are for entertainment, they may not include the nuance of an actual negotiated BDSM scene.