Meaning of -ER in English

-ER

-ere; akin to ·lat. -arius.

2. -er ·- a suffix used to form the comparative degree of adjectives and adverbs; as, warmer, sooner, lat(e)er, earl(y)ier.

3. -er ·- the termination of many english words, denoting the agent;

— applied either to men or things; as in hater, farmer, heater, grater. at the end of names of places, -er signifies a man of the place; as, londoner, ·i.e., london man.

Webster English vocab.      Английский словарь Webster.