Bouncing wave off tower:
Let's say the enemy has a gigantic minion wave to your normal one. Normally, the enemy minions will start pushing towards your tower, correct? Once the enemy minions get under your tower, your tower is going to very slowly kill the enemy minions one by one. Because of how long it takes, your minions will begin to build up. By the end of the event, usually, your minions have built up and the tower has killed the enemy minions. What started as the enemy minions pushing in your tower's direction is now your own minions pushing in the enemy tower's direction.
Resetting the wave:
Simlar to resetting the tower, it is killing all the remaining minions after a given skirmish. Let's say I get a solokill in mid lane, and all the minions are still there. If I leave them alone, the enemy laner has the opportunity to get most of those minions if they come back fast enough. What I want to do then is kill the enemy minions as fast as possible, so that my minions walk under the enemy tower, and I get the essentially free gold/exp while denying the opponent of said gold and exp. The reason why this is called "resetting" the wave is because if you do it fast enough, every minion in the lane will be dead before the next wave comes in, identical to how the game starts out: Ergo, the lane resets.
Both are tricks to help set up or counter pushes and shoves. Bouncing is usually done by champs with bad wave clear (Tryndamere without many items) while resetting is done by those with ample wave clear (Ziggs).