Voting machines must make sure that the votes they record are accurate. Nobody should be able to change your vote or otherwise rig the election.
One consideration when ensuring the accuracy of votes is preventing lost votes caused by machine errors that result in votes not being recorded. EVMs must be designed to prevent this. As such, voting machines will maintain separate backups of the votes they record. These backups can be separate electronic recordings of ballots, or they can be paper records that are printed when people use the voting machine. These backups can be used if there is a recount of the election.
Good security is an important part of making sure that votes are counted accurately. The aforementioned backups of votes help with this, as they can be used to check that the election has been tallied accurately. In addition, voting machines can be designed to detect and prevent attacks such as ballot stuffing (casting extra, illegitimate ballots to try to change the outcome of an election).
In addition to keeping votes accurate, they must be kept secret. For more information about that, see our privacy page. Making sure that votes are recorded accurately even while keeping them secret is a challenge. Without keeping votes secret, it would be extremely easy to verify results, because everyone would be able to simply look up their own vote and see that it was recorded correctly. However, as discussed above, making it possible to see who a particular person voted for comes with its own problems. As such, we use other methods of verifying the accuracy of votes.
The security measures programmed into voting machines are an important way of making sure that votes are not changed. One common additional strategy is a voter-verified paper audit trail, meaning that voting machines create paper copies of the ballots they record. Voters can then look at the paper ballot to see that it has correctly recorded their vote. The paper ballots can then also serve as a paper backup of the electronic votes cast.