This is the 99th most frequent Spanish word.
estaba
"Estaba" is the imperfect past tense of "estar," meaning "I/he/she/it was" or "was being."
The word 'estaba' is used here to indicate a non-specific past action or state, displaying the imperfection of the action of being at the store.
Estaba en la tienda cuando empezó a llover.
I was at the store when it started to rain.
The word 'estaba' here shows the ongoing past action, setting the background for another action that interrupts it (calling).
Ella estaba escribiendo una carta cuando llamaste.
She was writing a letter when you called.
The word 'estaba' is describing a past continuous emotional state, specifying something that was felt as ongoing or true in the past.