Charter schools are private schools with a public enrollment program. Instead of collecting tuition from parents, students who enroll are funded at a government-determined rate paid for by the government. It is intended to be a private alternative to public schools.

The concept is that charter schools provide competition to public schools which results in higher quality education for everyone. When a charter school is run better than neighboring public schools, enrollment will be high, the private venture will be profitable and public schools will improve by by adopting the standards and practices of the successful charter school.

The best case scenario does happen. There are some high performing charter schools and sometimes that does pressure neighboring public schools to perform better.

In practice, what generally happens is that the charter schools spend tuition on gimmicks that attract parents and students and they academically underperform. Since public schools are also funded by the number of enrolled students, when they experience a decrease in enrollment they get a decrease in funds. Poor performing schools with even fewer funds perform even worse.

Sometimes this results in a type of segregation where most of the good students attend the charter and bad students attend the public schools. The poor performing public school, having fewer quality students and less money will, surprise, get even worse.

More often charter schools underperform. In many states charter schools are typically founded by religion organizations that push secular requirements to and beyond legal limits.

They do vary wildly in quality. There are good charter schools, great charter schools, bad charter schools and straight up scam charter schools. They work well in some areas. On the whole they don't work well and cause considerable harm to neighboring public schools, but a good argument could be made that this is mostly due to poor oversight.

Another good argument could be made that they are doing exactly what they are designed to do by providing the kind of education that the community wants, even if it's substandard from the perspective of colleges and universities. The drawback of that argument is that it always comes as the cost of taking away funding for public schools harming children and parents who don't want religious organizations teaching their kids.

TL;DR: The charter school system is a chaotic shit show. Which is exactly what many people who wish for higher quality publicly funded education want and many people who wish to subvert and destroy the publicly funded education system want.