This pattern means that someone disregards or neglects one thing, person, or matter in favor of focusing on something else. It often carries a nuance of criticism or surprise at the imbalance of attention. The thing ignored is marked by は or を, and the pattern is used adverbially to describe the action that receives the attention instead. For example, 勉強はそっちのけでゲームをしている implies the person is playing games while ignoring studying. The choice between は and を is largely stylistic, with は giving a slight contrastive nuance. This pattern is quite formal and literary, usually found in written narratives, news articles, or formal speeches, not in casual conversation. In speech, you might hear something like ～をほったらかして, but that is a different construction. Note that the pattern does not require a subject; it simply describes the situation.