A capacitor dampens changing voltage levels like a shock absorber on an automobile.
The difference is that of "soda pop" and "Coca-Cola"
5V is a popular voltage for the WS2812/Neopixel.
Between 300 ohm and 500 ohm. It limits the current OUT of the Arduino and INTO the Neopixel (too much current will cause damage). 330 and 470 are common values (500 is not a common value).
Read the Adafruit Neopixel U:berguide.