"I read a book during the meal."
Translation:Eu leio um livro durante a refeição.
There is no good answer to "why". Gender is an inherent property of nouns - they just are, and it's up to us to learn which is which. In general, nouns ending in -o are masculine, but there are many exceptions, and -ção is one. It is an ending that has the same meaning as the English -tion, and often Portuguese words ending in -ção map directly to English words ending in -tion, though not here. The ending is (almost?) always feminine, so remember to check for it before falling back on "-o means masculine."
(If you study any other languages, Portuguese -ção maps to Spanish -ción and Italian -zione, both of which are also feminine endings, though you need only look at leite/leche/latte to see that gender is not always consistent among Latin's children. )