On several occasions since winning the election, President Trump has claimed that millions of votes were cast illegally in the 2016 election. He cites no reliable evidence. Numerous studies have found organized voter fraud rare.
Trump seems to conflate voter fraud with voter registration fraud. They are different things. Voting is a two-step process. First, one registers to vote, then one casts a ballot. The first step needs reform to ensure the integrity of the second.
People often register through voter registration drives organized by groups that pay their employees per sign-up. This creates an incentivize for those employees to fill out fraudulent registrations.
A 2012 Pew Research Center study found that roughly one in eight voter registrations is out of date, inaccurate or a duplicate. That's 24 million people. This includes 1.8 million people who are dead and nearly 3 million who are registered in more than one state.
People move and change their names. Often they don't notify their state's voter registrars when they do. When people die, their names may stay on voter rolls if their families say nothing, as many do.