According to the author of 2 Chronicles, Solomon was a very rich and powerful king who collected many chariots and horses, which were like the fancy cars and soldiers of his time. He kept them in special cities and with him in Jerusalem to show his strength and readiness to protect his kingdom.
2 Chronicles 1:14: And Solomon gathered chariots and horsemen: and he had a thousand and four hundred chariots, and twelve thousand horsemen, which he placed in the chariot cities, and with the king at Jerusalem.
This verse warns against multiplying horses, while 2 Chronicles 1:14 describes Solomon accumulating many horses and chariots.
Deuteronomy 17:16: But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as the LORD hath said unto you, Ye shall henceforth return no more that way.