This is the 10373rd most frequent Polish word.
ciężkich
"Ciężkich" means "heavy" or "difficult" in English, depending on context (genitive plural or accusative plural form).
In this sentence, 'ciężkich' is used as an adjective describing 'rzeczy' (objects), meaning 'heavy.'
Przenoszenie ciężkich rzeczy wymaga dużo siły.
Carrying heavy objects requires a lot of strength.
The word 'ciężkich' here is used as an adjectival modifier meaning 'heavy' or 'challenging,' describing 'chwil' (moments).
Ciężkich chwil nie można uniknąć w życiu.
Heavy moments cannot be avoided in life.
Here, 'ciężkich' is used as an adjective meaning 'heavy or robust,' describing 'zawodników' (players).