A power chord is the simplest chord there is: just the root and the 5th, often with the root doubled an octave higher. It's written with a 5, like E5 or A5.
Remember what makes a chord major or minor: its 3rd. A power chord has no 3rd, so it's neither. That makes it fit almost anywhere, in major and minor songs alike.
It also explains why rock guitarists love it. With distortion (the gritty sound of a rock amp), full chords quickly sound muddy: distortion creates extra notes out of combinations of the notes you play. With just the root and the 5th, those extra notes fit the chord (one even lands an octave below the root, making it sound bigger). Add a 3rd and they come out slightly off and clash. So a power chord stays clear and punchy, however much distortion you use.
A power chord is one small : the root on the low E or A string, the 5th two frets up on the next string, and the octave on the string after that, on the same fret as the 5th. Your index finger (1) plays the root, fingers 3 and 4 the other two notes. The same shape works on every fret, so you can play any power chord by just moving it.
At the open position the root is the open string, so fingers 1 and 2 play the other two notes:
A riff is a short, catchy pattern that repeats. Slide the shape along the low E string, from the open position up: E5, G5, A5, and back to E5. That's already a classic rock riff.
Try the same idea with other roots: any note on the low E or A string can start a power chord. The fretboard tool helps you find them.
That's the end of the course so far. Keep practising with the tools: build your own progressions in the progression builder, explore scales and chords on the fretboard, and learn the notes by heart with the note trainer. Or go back to the start.