‘ And the priests who bore the Ark of the covenant of YHWH stood firm on dry ground in the midst of Jordan, and all Israel passed over on dry ground, until all the nation were passed clean over Jordan.'
The priests then marched with the Ark into the middle of the Jordan and stood there on dry ground (ground on which there was no standing water) while all the people crossed over and safely reached the other side of the Jordan, having no doubts because they were confident that YHWH Himself was holding back the waters. They had arrived in Canaan!
“Passed clean over Jordan.” Literally ‘were finished to pass over', thus had finished passing over.
For the meaning of ‘dry' compare Genesis 8:13-14 where in Joshua 3:13 the ground was no longer covered in water and in Joshua 3:14 it had dried out. The noun here relates to the first.
“The ark of the covenant of YHWH.” The longer appellation stressed that it not only at this time reminded them of their covenant with YHWH, but also that that covenant was with YHWH, the God Who does wonders.