This is the 4625th most frequent Russian word.
детстве
"Детстве" means "childhood" (prepositional case in Russian).
The word 'детстве' is used here to specify the time period in which the action of loving to read fairy tales happened.
В детстве я любил читать сказки.
In childhood, I loved reading fairy tales.
In this sentence, 'детстве' is referring to someone's period of life as a child, used in the context of memories.
Она всегда вспоминает о своем детстве с теплотой.
She always remembers her childhood fondly.
Here, 'детстве' establishes the timeframe for the repeated action of bike riding.