This is the 2327th most frequent German word.
kleinen
"Small" or "little" (dative or accusative plural/masculine singular of "klein").
The word 'kleinen' is the adjective 'small' used here in the accusative case, agreeing in gender, case, and number with 'Hund' (masculine, singular).
Ich sehe den kleinen Hund im Garten.
I see the small dog in the garden.
Here, 'kleinen' is the adjective 'small' in the dative plural form, agreeing in number and case with 'Kindern' (plural).
Wir spielten mit den kleinen Kindern.
We played with the small children.
The term 'kleiner' is the adjective 'small' used in agreement with 'Garten' (masculine singular nominative) to describe it.