It's long and complicated but basically she was put under a conservatorship in 2008- it was after she locked herself in the bathroom with her son after an access dispute with her ex. You might remember pics of her being taken away in an ambulance (this was after the 2007 head shaving).
It means that her father (who is the main conservator) has control of her as a person and her as a brand. He has control over all her money. It's usually put in place for people with dementia etc who are totally incapable of making decisions for themselves.
Over the years, Britney has tried to get out of it (she spoke about it in her 'for the record' documentary, calling it worse than prison) but she hasnt been successful. It was only supposed to be temporary when she was having problems in 2008. It has meant that her dad (who by the way has a history of abuse and most recently he was reported for being violent with Britney's son) can pay himself (and other people) a wage to be her conservators. There's also a leaked voice mail where she she's telling someone that her dad is threatening to take her kids away. K Fed currently has custody.
The worst part is because of the restrictions, she cant even choose her own lawyer, so it's hard for her to get out of it.
The details are murky because Team Britney are so strict about who talks to her (eg she cant be alone with someone who isnt approved). Most notably, she spoke about the c'ship on Jonothan Ross and it was cut out. Many people who have worked with her during the c'ship have spoken about how she seems on edge and her people control everything.
There's obviously the mental health situation too which no one knows the truth about.
But at the base of it, her fans are saying if she is so incapable of controlling herself - deciding where she wants to go, driving her car etc etc, then why has she spent all these years since 2008 working (world tours, albums, products, vegas residencies). There is an abuse of power somewhere.
If you're interested in the proper explanation, BritneysGram started it all with a podcast episode a year or so ago called FreeBritney (its number 70 something) - they were approached by an ex paralegal working on the conservatorship case and wanted to reveal that one of the conservators quit because of how immoral it was. And it snowballed from there. This was longer than I intended!