Answer:
A dictionary
Step-by-step explanation:
Dictionaries are the best sources that can be used to understand the meaning of a difficult or not commonly used word.
Some dictionaries (usually virtual dictionaries) also offer the origin, part of speech, and synonyms of words which can help to enrich the understanding of a specific term.
For example; according to Merriam Webster, saturnine is an adjective that refers to something distant, cold or that acts slowly.