The sentence:

“Assert that nudity is essential to attain liberation”

means:

> They are claiming that being completely without clothes (nude) is a necessary condition for achieving moksha (liberation).

In very simple words:

• “Assert” = to strongly say or claim.
• “Nudity is essential” = being without any clothes is must, not optional.
• “To attain liberation” = to become free from all karmas and the cycle of birth and death (moksha).

So the full meaning is:

> “They strongly claim that a person must be nude in order to attain moksha.”

In Jain context:

• Digambar tradition: teaches that for a true, full monk, total renunciation includes giving up clothes; so for them, nudity is an essential part of the highest monastic life, which is the direct path to liberation.
• Shvetambar tradition: does not accept nudity as compulsory; they say clothes do not stop liberation, what matters is inner purity, right faith, right knowledge, and right conduct.

So, the sentence is describing a belief (mainly of Digambar monks) that without complete renunciation of even clothes, one cannot reach the final stage of liberation.