The term "credit for time served," or "CTS," can apply to different aspects of sentencing.
As a sentence itself, credit for time served means that the court will give the accused credit for the time they have served in jail pending bond or pending trial, had the accused not bonded out of jail. The problem is that a CTS sentence usually involves an adjudication of guilt, or a conviction. If the charge is a felony, then you can become a convicted felon just by accepting a CTS sentence.
If you are sentenced to CTS, you will not serve any more time in custody than what you have already served.
In broader terms, though, credit for time served is used to calculate how much time you will get credit for if you are serving a jail or prison sentence.
Florida law mandates that "the court imposing a sentence shall allow a defendant credit for all of the time she or he spent in the county jail before sentence." Florida Statute Section 921.161(1).
So if you are closing out your case pursuant to a plea agreement and the plea agreement states that you will serve a length of time in jail or prison, you are statutorily entitled to receive credit against your sentence for any time that you have already spent in jail. Time spent on house arrest or while out on a bond does not count as credit for time served. It is only time spent in jail.
So if you are sentenced to 364 days in county jail and you have been in jail for 75 days, you will be sentenced to 364 days with credit for 75 days. That means that your actual sentence will be 289 days (364 - 75 = 289).
Credit for time served and gain time are two completely different concepts. Gain time is time off your sentence that you acquire while you serve your sentence. Credit for time served is time off your sentence for time that you have already served.
If you are facing a probation violation and you resolve your case, you can receive credit for time served for all time served under that case number. For instance, you are eligible for all credit for time served acquired on the underlying case as well as all credit for time served acquired while being held on the violation.