'oH is a pronoun referring to a single thing which is not generally considered a being and/or not generally considered to be capable of speech. It is usually translated as "it".
In Klingon there is no verb "to be". Verbs that look like adjectives (often called "verbs of quality" or "stative verbs") already include "to be" in them. But to say that two nouns (or noun phrases) are equivalent, you have to use a pronoun as if it was both the subject and verb of the sentence:
paq 'oH "It is a book."
pong 'oH "It is a name."
pongDaj 'oH "It is her name."
mara 'oH "It is Mara". (Which might be taken as an insult if you are talking about the person and not about the name.)
If you also want to state what "it" is and equate two actual things, then you need to add in an explicit subject. However, the pronoun is already acting as both subject and verb, so you can't really add in the subject. Except that there is a very special and unique bit of Klingon grammar which allows you to use the "focus/topic" suffix -'e' to add in a duplicate subject for this kind of sentence only.
Thus, "Her name is Mara," is mara 'oH pongDaj'e'