WAYMARK (צִיּוּן, H7483, cf. Arabic suwwah, “guidestone”; Syrian swayâ, “stone-heap”). A sign set up along a path or route of travel.
A heap of stones, to mark a route of travel (Jer 31:21), used in this reference to mark the route of exile, so Israel could return the same way.
A pile of stones, which is a monument to mark the burial place of the dead (Ezek 39:15, “sign,” 2 Kings 23:17, “title” [KJV]). A pillar might be used (Gen 35:20; 2 Sam 18:18), or a heap of stones, piled over a tomb (Josh 7:26; 8:29); or a stele or maṩṩebah.
International Standard Bible Encyclopedia (1915)
In Jer 31:21, "Set thee up waymarks," explained by the parallel, "Make thee guide-posts" (the "Make thee high heaps"). A sign or guiding mark on the highway.