"To shrive" -- the Roman Catholic practice of confessing sins to a priest.
"To shrove" -- the Roman Catholic practice of the priest imposing penance and granting absolution.
The Old English word "shrive" comes from a few different roots (as many other English words do). One root means, "to write down" (as in, to give specific limits to something; think: "script"). Another means, "to cut away" (as in, to remove that which is unwanted; think of "proscribe"). The result in ancient English is that the word "shrive" became the word used for the practice of confession/penance/absolution (though not, as far as I can tell, for the sacrament itself). In fact, in some places in ancient England the Catholics would refer to "pre-lent" (the "gesimas") as "Shrovetide"!
So, how did that become a "pancake supper", you may ask? The "shriving" (as in "giving limits") of the Church bell on Shrove Tuesday called people to the Church to make a confession right before the season of Lent begins, and then they would celebrate the joy (yes, joy) of Lent beginning the next day with a final feast before Easter. They would give thanks for what God had provided them, which made it all the more vivid to give those same things up (eating plain food, and more simple meals) during Lent.
Lent is supposed to be an austere time of self-sacrifice. We are supposed to "give up something for Lent" because that is what spiritual rigor is all about (if we cannot give up small things, we will never give up our sins). A hearty meal of sweet pancakes and sugary toppings is a way to remind us of the meager fair that many submit themselves to during Lent. Although we are not required to eat more simply during Lent (maybe we will be again someday?), there is nothing stopping any Catholic from choosing foods that are more plain to remind them that Lent is not "just another time on the calendar", and it always has a spiritual benefit if we practice it with faith in God.
Shrove Tuesday is not (as in: NOT!) Mardi Gras (or Fat Tuesday), which was a celebration to "get in" as much sin as possible before the priests started paying closer attention during Lent. This is why, obviously, modern Mardi Gras celebrations in some places like New Orleans are just festivals of debauchery that glorify immorality and have no respect at all for the practices of Lent. It is a further sign of the confusion that exists today when Catholics have a "Mardi Gras" celebration [please do not do it!].
Make sure, then, that you shrive (i.e. get ready for Lent). If you cannot go to confession today, at least go as soon as possible as a proper beginning of Lent. As a part of your shriving make sure that you choose a lenten sacrifice; something that you will willingly give up during Lent. In sacrificing our joys during Lent, we can better recall the sacrifice that Christ made for us, and when we do it collectively, the spiritual impact on our community has even greater blessings that come from it. Lent begins tomorrow; get ready now.