This is the 1596th most frequent Russian word.
счастье
Happiness; joy; bliss.
The word 'счастье' means 'happiness' in this sentence, and it is used as a noun representing abstract contentment or joy.
Счастье заключается в мелочах.
Happiness lies in the little things.
The term 'счастья' is the genitive form of 'счастье', denoting the wish for someone to have happiness.
Я желаю вам счастья и успехов.
I wish you happiness and success.
The word 'счастье' is a subject noun here, used to define what constitutes happiness for an individual.