The command given to Joshua marks a profound test of faith, echoing the great miracle of the Red Sea. The priests carrying the Ark of the Covenant are told to step directly into the flowing river before any miracle occurs, requiring absolute trust that the waters will part for them. This directive is meant to show the nation that God supports Joshua just as He supported Moses in the past [אלשיך].
Once the priests enter the river, there are different perspectives on exactly where they are supposed to halt. One view suggests they stop immediately at the water's edge on the eastern side, just after clearing the thick reeds on the riverbank [רש״י, ביאור שטיינזלץ, מלבי״ם]. Another perspective maintains that the priests cross the entire dried riverbed, stopping only at the far western edge right before exiting onto the other side [מצודת דוד, רד״ק].
Regardless of the exact location, the priests are required to remain standing on the newly formed dry ground within the riverbed rather than continuing out of the water [חומת אנך, מצודת דוד, ביאור שטיינזלץ]. This pause is crucial because the waters of the Jordan are held back entirely by the presence of the Ark. If the priests were to exit the riverbed, the waters would instantly rush back to their natural place. Thus, they must hold their position until the entire nation completes the crossing in safety [רש״י, מלבי״ם]. While the priests maintain this steady position, and before they finally step out of the river, twelve stones are gathered from the dry riverbed, and another set of stones is erected in their place [רד״ק].