This is the 4527th most frequent Turkish word.
ağzına
"To/into/at/on his/her/its mouth."
The word 'ağzına' indicates possession and direction, describing to whose mouth the chocolate was applied.
Çocuk ağzına çikolata sürdü.
The child smeared chocolate on his mouth.
Here, 'ağzına' is part of the idiom 'ağzına sağlık' ('health to your mouth'), used to compliment someone's speech.
Ağzına sağlık, çok güzel anlattın.
Well done, you explained it very well.
In this context, 'ağzına' indicates where the food was placed, specifying the cat's mouth.