Refers to the back or rear area of something, often used in spatial or geographical contexts. Can also be used as a prefix in compounds.
The enemy forces attacked from the rear.
There is a small village behind this mountain.
doggy style; rear-entry position
hinterland
back swamp; back marsh; back slough
Compound of 後 (behind, after) and 背 (back). The exact historical derivation is uncertain, but the kanji combination naturally conveys the meaning of 'rear area'.