The N-Declension in German grammar is a rule applied to the declension of certain nouns, mainly masculine ones, predominantly ending in -e, -ent, -ant, -ist, -or, -ein, etc. This rule affects the accusative, dative, and genitive cases, marking these nouns with an extra -n or -en ending in all forms except the nominative singular. Understanding N-Declension is crucial for German language learners as it is consistently used in written and spoken German.