In a dual-class structure the public buys a low-vote class while founders keep a high-vote class. Economically the classes are usually equal; politically they are not. A founder holding 8% of the shares can control 55% of the votes.
For a trader this changes two things. A proxy-fight or activist-investor campaign is close to impossible, which removes one common catalyst. And some index providers restrict or exclude dual-class companies, affecting index-inclusion demand.
Example: 90M Class A shares at one vote and 10M Class B shares at ten votes gives 90M plus 100M equals 190M total votes. The Class B holders own 10% of the company and cast 53% of the votes.
Related: share-class, super-voting-shares, voting-rights, activist-investor