"Naman" is a versatile enclitic and may denote various things like sympathy, disgust, impatience, and plea. It doesn't have English equivalent. It implies a lot of emotions depending on the context and tone of the speaker - making it very important in Tagalog language but pretty tough to explain. Hm... let me give you examples of its uses.
For coordinating
--- Ako bahala dito. Ikaw naman doon. = I'm in charge here. While you, there.
--- Kumusta ka?... Eh mommy mo, kumusta naman? = How are you? Uhm you're mommy, how's she?
"Actually"
--- Mabait naman siya. = He's actually kind.
* Probable emotion behind: consider his kindness.
"Again" = Na naman
--- Ikaw na naman? = You again?
* Probable emotion behind: I can't believe this.
For emphasis or superlative adverb
--- Ang pangit naman! = How (very) ugly!
* Probable emotion behind: Ew. I'm really disgusted.
--- Kawawa naman. = How pitiful. :(
* Probable emotion behind: I agree/ sympathise with them.
--- Oo naman. = Of course OR Yes, I agree
* Probable emotion behind: I agree with your sentiment or opinion.
For emphasis in request or order.
--- Kumain ka naman. = Please eat. / Eat, will you?
* Probable emotion behind: : Have mercy, please eat. Mothers usually say this to their stubborn children who'd rather play than have their meal.
Vague / modest replies
--- Hindi naman. = No (not really).
--- Mabuti naman = Good / Fine (somehow).
* Probable emotion behind: Some where in between, better read between the lines.
Expression
Naman! = Wtf, For godssake, Have some consideration, etc...
* Probable emotion behind: I'm pissed.
Other helpful phrases:
Bakit naman? = Why is that?
Huwag naman. = Please don't
Grabe naman. = That's too much. (Got to have some consideration)
Generally naman is just used to add gravity to the emotions of the speaker.
Hope this helps. :)