And God said, "Let there be light,"
— Genesis 1:3

Lighting two candles separates time periods: from regular days to festival days.

בָּרוּךְ אַתָּה אֲדֹנָי אֱלֹהֵינוּ מֶלֶךְ הָעוֹלָם אֲשֶׁר קִדְּשָׁנוּ בְּמִצְוֹתָיו וְצִוָּנוּ לְהַדְלִיק נֵר שֶׁל [שַׁבָּת וְשֶׁל] יוֹם טוֹב

Blessed are you, God, ruler of the universe,
who makes us holy with your good rules such as lighting the candles of (Shabbat and of) a good day.

haggadah Section: Introduction
Source: The Minimalist Haggadah by Jon Kessler