This is the 12842nd most frequent Polish word.
nudne
Boring.
Here, 'nudna' (a declined form of 'nudne') is used as an adjective describing the feminine noun 'lekcja.' It indicates that the lesson was uninteresting.
Lekcja była nudna i trudno było się skupić.
The lesson was boring and it was hard to concentrate.
In this instance, 'nudne' functions as an adjective describing the plural noun 'filmy' (movies), showing that the movies are uninteresting.
Te filmy są nudne; wolę coś bardziej ekscytującego.
These movies are boring; I prefer something more exciting.
Here, 'nudnych' (a declined form of 'nudne') modifies the plural noun 'zajęć' (activities), emphasizing the uninteresting nature of such activities.