תָסוּר, מִכָּל-הַדְּבָרִים אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם
הַיּוֹם--יָמִין וּשְׂמֹאול: לָלֶכֶת, אַחֲרֵי אֱלֹהִים
14 and shalt not turn aside from any of the words which I command
you this day, to the right hand, or to the left, to go after other gods
to serve them.
The Torah sets out a condition for observance of the Torah, and this is mentioned several times, including in the Blessing of Devarim 28.
Turning aside from the Mitzvah of the Torah - to the right or the left - is forbidden.
It suggests that the Karaite position is precisely on the line of the written torah, and should not veer to the right or the left.
The "right" is implying stronger, and thus the Rabbinic type movements who added many "fences" and strictures to "strengthen" the observance, whilst the "left" implies weakening, so it could mean secularism, Reform, or Reconstructionism. Karaism should therefore maintain its position on the correct path of the written torah, without adding extra laws ans strictures, and also to avoid reformist tendencies who permit anything that is fashionable.