This verse is spoken by the Rabshakeh, a high-ranking official of the Assyrian King Sennacherib, to the representatives of King Hezekiah of Judah. He's using a combination of propaganda, insult, and threat to demoralize the Judaeans and convince them to surrender without a fight. Let's break it down:

"Now therefore, please give pledges to my master the king of Assyria..." This is the core message: submit to Assyrian rule, become a vassal state, and pay tribute.
"...and I will give you two thousand horses..." This sounds like an offer of assistance, but it's a sarcastic one. Horses were crucial for warfare, particularly for cavalry.
"...if you are able on your part to set riders on them." This is the key to understanding the true meaning. The Rabshakeh is insulting the Judaean army. He's saying, in effect, "Even if we gave you two thousand horses, you wouldn't have enough skilled soldiers (riders) to use them effectively. Your army is so weak that you can't even man that many horses."

In essence, the Rabshakeh is saying:

"Surrender to Assyria. Even if we gave you horses, you're too weak to defend yourselves. You lack the manpower and military strength to resist us."

The purpose is multifaceted:

Demoralize the Judaeans: By undermining their confidence in their own military capabilities, the Rabshakeh hopes to break their will to resist.
Undermine King Hezekiah's authority: If the people believe they are helpless, they might turn against Hezekiah and urge him to surrender.
Avoid a costly siege: If the Judaeans surrender without a fight, the Assyrians can take Jerusalem without wasting resources and manpower on a long and difficult siege.

The Rabshakeh's speech is a classic example of psychological warfare, designed to achieve a swift and bloodless victory for Assyria.