A slip stitch is a stitch that you slip on to a working needle without having to knit or purl. You do this by slipping a stitch from a left hand needle to a right hand needle (for right-handed knitter). That's all to it.
They are two ways you can slip a stitch - Purlwise or Knitwise. If the instruction doesn't tell you which way you should slip your stitch, you can do it either way but keep them all the same way.
I made a little video for you to see the differences between a purlwise-slip stitch and a knitwise-slip stitch.