This is the 1292nd most frequent German word.
gutes
"Good" (neuter form of "gut").
The word 'gutes' is the neuter singular nominative form of the adjective 'gut' in its declined form, modifying the noun 'Buch'.
Ich suche ein gutes Buch.
I am looking for a good book.
Here, 'gutes' takes the neuter singular nominative form to describe the noun 'Angebot'.
Das ist ein gutes Angebot.
That is a good offer.
The word 'Gutes' is used substantively, meaning something good, where the adjective 'gut' is turned into a noun in neuter singular form.