Since you originally placed this question under “Law,” I assume that you are asking about reentry in that context and not in the context of space travel. If that is correct, then reentry is the process of allowing a person who has been in prison to reenter society. That process is not always effective in the United States today.
A person who has been in prison can have a hard time reintegrating into society. The person may have been away in prison for a relatively long time. They may not have much in the way of skills. They may be at high risk to reoffend. Therefore, reentry is very important. If reentry is not handled well, ex-convicts may reoffend soon and end up back in prison. This is not a good outcome for society. Therefore, it is ideal if reentry is handled in such a way that the offender has a chance to become a contributing member of society.
The problem is that this is very difficult. Many released prisoners have few skills and are not good prospects for jobs, particularly with their records. Programs that are meant to provide them with skills are both expensive and politically unpopular. Even if these programs do exist and are funded, they are not always going to be effective. Therefore, it can be very difficult for reentry to be successful and for the ex-prisoner to remain out of prison for the long term.