¿Qué significa Voorletters ?

See a translation

Yes, 'voorletters' refers to the initials of all your given names. Barak Obama is actually called Barack Hussein Obama, so his initials would be B.H. :-) The first letter of your family name is NOT part of your 'voorletters'.
If you only have one given name (like most Japanese people, I expect), then you only have one initial. I expect Dutch people on average have two given names, so two 'voorletters'.

'Tussenvoegsel' refers to the small word(s) that precede a lot of Dutch family names. They are usually prepositions and articles, and are written in small case.
Examples:
de, van, 't, van 't (literally: the, of, the, of the)
Note that if somebody's last name is 'de Boer' (literally: the Farmer) we would think of their last name starting with a B, because we ignore the 'tussenvoegsel' when alphabetising.