This law. Probably the actual legislation, not the blessings and curses later enumerated in vs. 14-26 (see PP 500). The word here translated “law” is torah, a general term for all sacred instruction, and not confined to the Decalogue. Here it refers specifically to the will of God as revealed to Moses.
This was to be written legibly, and conspicuously set up in a central location, a perpetual reminder of the covenant between God and Israel (see Joshua 8:30-35). Today we have God’s revealed will in our homes, but Israel had no such privilege as this. Accordingly, the very stones of the countryside were to proclaim it for all men to read, lest they forget.