This is the 2121st most frequent Polish word.
opiera
"Opiera" means "rests," "leans," or "is based (on)" in English, depending on context.
In this sentence, 'opierać się' is used metaphorically to signify resisting or opposing something such as temptations.
Musisz się opierać pokusom.
You must resist temptations.
In this example, 'opiera się' is used to indicate a physical action of leaning against an object.
On opiera się o ścianę.
He leans against the wall.
Here, 'opiera się' means that the theory is founded or supported by facts, indicating reliance or basis.