Jennah- (I love that name by the way...I'm going to name one of my daughters Jennah someday, and another Malaak(Angel). InshAllah)
As brother Rashid has already mentioned, abrogation in the Qur’an basically indicates certain verses were only applicable during a certain period of time. These verse were later overwritten by others which we should follow today. One of the most common example is the gradual prohibition of alcohol.
At first Allah(swt) warned us about the negative affects of alcohol, but did not completely forbid it.
Verse (2:219) They ask thee concerning wine and gambling. Say: "In them is great sin, and some profit, for men; but the sin is greater than the profit." They ask thee how much they are to spend; Say: "What is beyond your needs." Thus doth Allah Make clear to you His Signs: In order that ye may consider
In the next stage, use of wine was prohibited before the times of prayer to insure that no prayer time would arrive without everyone being completely sober. Allah says:
Verse (4:43) O ye who believe! Approach not prayers with a mind befogged, until ye can understand all that ye say,- nor in a state of ceremonial impurity (Except when travelling on the road), until after washing your whole body.
The absolute prohibition of wine came after the people were prepared for it. After developing this gradual course Allah(swt) prohibited it completely in Surah Al Mai’dah…
Verse (5:90-91) O ye who believe! Intoxicants and gambling, (dedication of) stones, and (divination by) arrows, are an abomination,- of Satan's handwork: eschew such (abomination), that ye may prosper.
Satan's plan is (but) to excite enmity and hatred between you, with intoxicants and gambling, and hinder you from the remembrance of Allah, and from prayer: will ye not then abstain?
As you can see the final verse abrogates the other 2.
This can also apply to Hadith. A sahih hadeeth is one that is authentic. This means that we are pretty sure this is in fact the words of the Prophet(saws). Whether it is overwritten by another hadeeth that comes later or not, doesn’t invalidate this one nor does it render it unauthentic. Both can be Sahih hadeeths, but one might no longer be applicable today. It’s useful in the sense, that it helps us put a timeline to certain events and possibly, give us a reason as to why the previous ahadeeth were abrogated. For example, the hadeeths that support the first verses about alcohol being bad for the health and narrate incidents about it being permissible, can be Sahih. This hadeeth however, would not be suitable guide now. It would be overwritten later by another hadeeth of the Prophet(saws) emphasizing the prohibition of alcohol.