Originally it means "facing (something, somone or the other side)". But in modern Hebrew מול is used for this meaning, physically; in some contexts both מול and נגד are used, e.g. in sport matches (English "versus"). נגד suggests a more sharp conflict; we would use only נגד and not מול for opposing sides in wars, even though it's conceptually pretty much like a sport match - only more grave.
From en.wiktionary.com about the word נגד :
Verb: נָגַד (nagád) (dated) to oppose
Noun: נַגָּד (nagád) warrant officer, resistor (electronic device)
Preposition: נֶגֶד (néged) against, opposite
I am against = אני נגד (אֲנִי נֶגֶד - ani neged) or אני מתנגד/ת (אֲנִי מִתְנַגֶּדֶת - ani mitenagedett)