Scratch

Abs is short for absolute value, meaning the distance from zero, or simply, it makes a value always positive, so abs(-5) is 5 and abs(5) is 5.

Sin is a trigonometric function, in a right triangle it means opposite/hypotenuse.  The number given is an angle and the sin function will give you the ratio of those two sides.  It's wave-like graph function also makes it useful for things like making an object float up and down smoothly.  Sin, cos, tan, and such are pretty complex, and might need a bit more explaining, here's the absolute basics to it, so you get what it is and does.

Give me a second and I'll also give an overview of velocity.

If you want a good tutorial on velocity for platformers (side view movement), here's one you can look at, just click the velocity and sensing tabs.