Sound like he was on probation and he violated. Don't know how serious until you learn what the alleged violation is.
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VOP means violation of probation. Detain in this context means arrest. The F means felony. The other numbers appear to be a case number and the number of a statute. Since its a felony VOP he could be facing some jail time. Consult an attorney in your area.
He is being detained for a probation violation. This detainer was issued by his agent, not a judge, and if they determine he violated his probation he may be held without bond pending the outcome of revocation proceedings. If he is revoked, what happens next depends upon his original sentence. If sentence was imposed and stayed, he will simply begin serving the previously imposed sentence. If sentence was withheld, he will remain in jail without bond until returned to court for sentencing. By definition, a felony is a charge that can result in a prison sentence, so yes, it is potentially serious.
By checking CCAP, you may be able to find his underlying conviction, and determine his potential exposure.
This answer is provided for general information only. No legal advice can be given without a consult as to the specifics of the case.