This is the 120th most frequent Spanish word.
dentro
"Within" or "inside."
In this sentence, 'dentro' is used as a preposition to denote a location within something.
El gato está dentro de la caja.
The cat is inside the box.
'Dentro' here functions as a time expression indicating 'in a little while' or 'soon'.
Dentro de poco llegarán los invitados.
Soon the guests will arrive.
Here, 'dentro' conveys a figurative sense of being inside one's emotions or thoughts.