This sentence may have begun in the previous verse, see the comments there.

He has come to Aiath: The pronoun He refers to the Assyrian army. The town of Aiath is also known as Ai. Although its precise location is unknown, it was clearly in the border area between the northern kingdom of Israel and the southern kingdom of Judah (see Gen 12.8; Josh 7).

He has passed through Migron: The town of Migron was near Michmash in the northern part of Judah.

At Michmash he stores his baggage: Michmash was a town about 11 kilometers (7 miles) north of Jerusalem. It was the location of important events in the life of King Saul, described in 1 Sam 13. For he stores his baggage, Good News Translation has “They left their supplies” (similarly Contemporary English Version, New International Version). Literally it says “he deposited his vessels/implements.” Both the Hebrew verb and its object here have a broad range of meanings, but it is clear that the Assyrians placed certain things there that had to do with their invasion of Judah. It is not clear why they left them there. The word baggage may be used but the Hebrew term here can also refer to weapons or other supplies. We may say “they left their loads” or “they arranged their supplies.”

For the translation of this verse consider the following examples:

• The Assyrians have reached Ai [or, Aiath];
they have crossed into Migron,
at Michmash they have left their supplies.

• The Assyrian army has come as far as Ai; it has crossed into Migron and at Michmash there is a supply dump.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .