Sliding up/down is used in some songs and is therefore not a technique that a beginner would usually use (other than making "funny noises" (little brother) with it).
Sliding up or down requires a note that is being played and a finger on the string on a fret.
up is when you play a note on one of the lower frets (for example fret 1
2 or 3) and, while keeping that finger pressed against the fretboard,
moving it upwards (closer to the body of the guitar) towards the higher
frets (for example fret 5 -> upwards).
Drop D has many uses. Basically you tune down your E string until it is D (DADGBe).
has a few purposes. It can make power chords much easier to play. For
example: playing an F power chord in Drop D would be listed as 333xxx.
Because since E has been lowered to D, you need to compensate by playing
on a higher fret to maintain the power chord. This is easier for
beginners because instead of using 3 fingers, you place one finger
across all 3 strings).
This can also sometimes save you from tuning down. There are a few songs where you are asked to tune a half-step down (Nirvana). But with some songs it is just because one string cannot be played because it is "below the nut".
To understand, take the G power chord: 355xxx. Slide it down to F#:
244xxx. And down to F: 133xxx. And then to E: 022xxx. You cannot play Eb
because the nut is in the way and basic logic states you tune a
half-step down. But tune to Drop D and play 111xxx. You will be playing
Eb without tuning your whole guitar.