This is the 1st most frequent Hungarian word.
a
The Hungarian word "a" is the definite article "the" in English.
Here, 'a' (or its variant 'az' to make it flow better with vowels) precedes 'könyv' to indicate it is a specific one.
Az asztalon van a könyv.
The book is on the table.
Here, 'a' is the definite article, used to specify 'nap' (sun).
A nap süt az égen.
The sun is shining in the sky.
Here, 'a' is again the definite article, specifying 'válaszadó' (respondent)'s viewpoint.