el fondo NOUN bottom; end
Fondo has two main meanings, bottom and end, so it’s another interesting case where Spanish uses one word, and English two.
El fondo de la cazuela estaba todo quemado. The bottom of the pan was all burnt.
en el fondo del mar at the bottom of the sea
From its idea of being at the bottom of something, en el fondo is used to say how you really think or feel:
Mayormente, en el fondo, los divorciados son siempre grandes románticos. Generally, divorced people are great romantics deep down or great romantics at heart.
The other main meaning of fondo is end:
Mi habitación está al fondo del pasillo. My room is at the end of the corridor.
Notice that the phrase al fondo de is the exact parallel of at the end of in structure.
To help you remember the link between the two meanings, both are the most distant point of something, but one is thought of vertically, = bottom, and the other horizontally, =end.