Never stop.

Meaning: This means to continue an action indefinitely and not come to an end. It's like saying, Continue an action indefinitely and not come to an end.

Choice of Words: Never is an adverb that means at no time in the past or future; not ever. Stop is a verb that means to come to an end or to discontinue an action. When you say "Never stop", it means to continue an action indefinitely and not come to an end.