Learn Spanish (Latin America): Su identidad es un secreto; por favor, no la revelen. - Her identity is a secret; please, do not reveal it.

In Spanish, pronoun placement depends on whether a command is positive or negative. For negative commands (when you tell someone not to do something), the object pronoun must go between the word no and the verb. So it is no la revelen. If it were a positive command, the pronoun would attach to the end of the verb: revélenla.

Does la mean it here? Why not use lo?

Yes, la means it. Even though secreto (secret) is the closest noun in the sentence, the speaker is really saying do not reveal her identity. Because identidad ends in -dad, it is a feminine noun, so we must use the feminine pronoun la. If it referred to the masculine word secreto, it would be lo.

Why does the verb end in -en (revelen) instead of -an (revelan)?

Revelen is the command form of the verb revelar (to reveal) for ustedes (you all). When you give a command to a group, regular -ar verbs swap their usual -an ending for an -en ending.

The translation says Her identity, but doesn't su also mean his or their?

Exactly! Su can mean his, her, its, their, or your (formal). Standing alone, this sentence could just as easily mean His identity is a secret or Their identity is a secret. The translation chose Her simply to give you one naturally correct meaning in English.