Joshua 4:20

‘And those twelve stones which they took out of Jordan, Joshua set up in Gilgal.'

This was in obedience to YHWH Who had told them that they must be set up at the place where they first lodged (Joshua 4:3). The stones had been carried there and laid there (Joshua 4:8), now Joshua erected them in a pile (or in a line, or even as a memorial altar) and declared their significance and importance for the future. The heap was a witness to the faithfulness of YHWH and His great power (compare Genesis 31:48). It indicated the border of the land and that YHWH watched over the land (compare Genesis 31:49; Genesis 31:52).