Negative adverb used before verbs, adjectives, and other adverbs to indicate negation; also used as a bound-form prefix to form terms with opposite meanings

to exceed a limit; to overstep a boundary; to surpass or go out of sequence

(literary) a boundary or restricted area; usually used in the phrase 不敢越雷池一步 bù​gǎn​yuè​léi​chí​yī​bù, meaning one must not overstep the mark