The two basic techniques of gerrymandering: packing concentrates an opponent's voters into a few districts they win by large margins, and cracking spreads them across many districts where they fall short.
In more detail
Both waste the targeted side's votes: surplus votes in packed districts and losing votes in cracked ones. Combined, they can give a party a majority of seats with a minority of votes. Measures such as the efficiency gap compare wasted votes between parties to detect such maps. In Rucho v. Common Cause (2019) the US Supreme Court held that federal courts cannot decide partisan gerrymandering claims, leaving remedies to state courts, legislatures and independent commissions.