לְמַ֗עַן תִּֽהְיֶ֛ה זֹ֥את א֖וֹת בְּקִרְבְּכֶ֑ם כִּֽי־יִשְׁאָל֨וּן בְּנֵיכֶ֤ם מָחָר֙ לֵאמֹ֔ר מָ֛ה הָאֲבָנִ֥ים הָאֵ֖לֶּה לָכֶֽם׃
that this may be a sign among you, that when your children ask in time to come, saying: What mean ye by these stones?
Read kabbalistic (mystical) interpretation of Joshua 4:6: and classical mystical sources.