A detainer is a formal legal hold placed on an inmate by a jurisdiction that has separate pending charges or unresolved business with that person. Finding one on parole paperwork means that even though she has been granted parole on the current sentence, she is not walking out the door into freedom. She is walking out of one legal situation and directly into another.
Here is what is actually happening. The county that issued the detainer has charges against her that have not yet been adjudicated, meaning she has not been brought before a judge to answer for them. That jurisdiction has essentially reserved the right to pick her up the moment her current sentence or parole status releases her. The detainer is their mechanism for making sure she does not slip through without facing those charges.
In practice this means that when she is released from the facility where she is currently housed, instead of going home she will be transferred to the custody of the county that filed the detainer. From there she will go through the booking and arraignment process for whatever those charges are, and the legal process starts fresh on that case.
The most important thing right now is finding out exactly what the detainer is for. Her attorney or the facility's case manager can tell her which county filed it and what the underlying charges are. That information determines everything about what comes next, how serious the new exposure is, whether bail is a possibility, and what kind of legal representation she needs to have in place before that transfer happens.
Do not wait on this. The time between learning about the detainer and the actual transfer is the window to get a attorney engaged in the new jurisdiction.