The Head Rest sex position is an oral sex position most commonly used to facilitate fellatio.
To get into this sex position, the receiving partner should lie on their side with their top leg lifted up and moved slightly back so their ankle is slightly behind their bottom leg. The partner performing the oral sex can then lie facing the receiving partner, resting their head on the receiving partner's bottom leg.
As this position leaves the receiving partner's hands free, it is an easy sex position to include sex toys into. For example, in the image above, the receiving partner is using a wand massager, like the LELO Smart Wand Medium or Satisfyer Wand-er Woman, to stimulate the performing partner's clitoris. A penis stroker or penis vibrator could also be used on the giving partner. Including sex toys means both partners are receive stimulation instead of just one!
The receiving partner's leg acts as a head rest of their partner which is how the position got its name.