chain throwing is simply using a certain throw in a certain character matchup to be able to grab your opponent before he can recover. Depending on the character matchup, how long you can chain throw (and if, and what throw you use) changes.
For example, marth can chain throw peach (unless she wants to DI into a tipper fsmash) from 0 to 30% damage. Sheik can chain throw… a whole lot of characters (pikachu and yoshi especially) with her down throw… in fact, she can chain pika and yoshi from 0 to around 70 if done correctly. peach’s upthrow can chain fastfallers (fox, falco, captain falcon) from somewhere in the 30% range to the 70% range (note that this will not work outside of this specific damage range) with her up throw.
not that chain throwing is easy mind you, it requires extreme timing. even if you managed to master chain throwing, you don’t do the EXACT same thing every time. See, while you hold them, your opponent is mashing the buttons and holding the control stick in directions to try to escape the grab itself. more importantly, the direction that they hold while you acually throw them will influence where they fly/bounce. In most cases, they can influence themselves to get behind you and make you turn around, or move different distances away from you to screw up the chain throw. also, you may have to quickly run and jump cancel grabs to continue the chain after the throw (start running in the direction they move, then hit the jump button and immediately after the grab button so you grab before you actually leave the ground - the net result is a slightly moving standing grab animation instead of the slower running grab animation). Also, chain throwing really requires you to be pretty much perfect in its execution to work - a slight mistake and they can either escape or hit you before you can grab them again. It also usually does only about 6-8 damage per throw, so you’ve gotta chain them quite a few times for this to do significant damage.
wavedashing is pretty hard to actually explain well without an example, but here goes. you start by hitting the jump button. however, before you actually leave the ground, you hold diagonally toward the ground and a direction (preferably very close to the horizontal, but enough that the controller registers that you’re partially holding down) and air dodge. Done correctly, you’ll slide across the ground doing the air dodge’s landing animation, which for most characters is faster than their actual dash. you can use this to move around quickly in either direction, without turning around, to set up attacks or get out of the way of things.
