Voir dire is a process where lawyers ask potential jurors questions to see if they're able to be impartial. Peremptory strikes are a tool that lawyers can use to remove a particular person from consideration as a juror. Both are valuable to make sure that the jury that's selected is able to decide the case based on the evidence, rather than based on preconceived notions.
The Sixth Amendment in the U.S. Bill of Rights guarantees the right to trial by a "fair jury." Voir dire is the process that's used to make sure that a fair jury is chosen. Potential jurors are asked questions about things that might make them biased towards one side or the other, and lawyers can then request that the judge disqualify jurors with biases.
Although all U.S. states use voir dire in jury selection, the process can be very different depending on the state. Some states limit voir dire to making sure that none of the jurors have personal connections to either side. Other states allow attorneys to ask questions that are meant to show underlying biases (for example, if a potential juror’s family member was killed in a car accident, that could bias them in a car accident case).
If the judge determines that there is a likelihood that a particular juror will be biased towards one side, that juror can be removed from the jury pool "for cause." This doesn't mean that the juror has done anything wrong; it just means they're not a good fit for that particular jury.
Peremptory strikes are a way for a lawyer to remove someone from the jury pool without showing cause. There are times when a potential juror might not have enough bias to be removed for cause, but the lawyer suspects that the juror would not be fair to their client.
Peremptory strikes do not need any justification for lawyers to use them. Because of this, only a certain number of peremptory strikes are allowed for each side. Once those strikes are used up, lawyers will need to convince the judge to remove jurors for cause.
Just like removal for cause, peremptory challenges don't mean that the potential juror has done anything wrong. It just means that one of the lawyers thinks they're not the best fit.
If it's a jury case, there will always be a voir dire process to ensure that Sixth Amendment rights are upheld. However, it's possible to go through the voir dire process with no challenges for cause and no peremptory strikes. That being said, in the vast majority of cases challenges for cause will be made, and in many other cases, peremptory strikes will be used as well. These tools are important for ensuring a fair trial for everyone involved.