This is the 898th most frequent Turkish word.
içi
The inside; interior.
Here, 'içi' refers to the interior or inside of the noun it is associated with, in this case, 'kutu' (the box).
Kutu'nun içi boştu.
The inside of the box was empty.
'iç-' in 'içinde' means 'in' or 'inside,' used as a locative case postpositional marker.
Yemeğin içinde baharat var.
There is spice inside the food.
'içi' metaphorically represents feelings or inner emotions.