This verse, part of Rabshakeh's speech in Isaiah 36, is a sarcastic challenge and a form of psychological warfare aimed at demoralizing the people of Jerusalem. Let's break it down:

"Now therefore, please make a pledge to my master the king of Assyria...": Rabshakeh, as the envoy of the Assyrian king Sennacherib, is urging the people of Jerusalem to surrender and become subservient to Assyria. He is trying to convince them that resistance is futile.
"...and I will give you two thousand horses...": This is the core of the sarcastic offer. Rabshakeh claims he could easily provide them with 2,000 horses. This number is likely exaggerated to emphasize Assyria's wealth and military power.
"...if you are able on your part to set riders on them.": This is the crucial punchline. Rabshakeh is implying that the people of Jerusalem are so weak and incapable that they wouldn't even have enough trained soldiers (riders) to use the horses. He's not genuinely offering them horses; he's highlighting what he perceives as their military inadequacy.

In essence, Rabshakeh is saying: "Don't even think about resisting us. You're so weak and your army is so pathetic that even if we gave you horses, you wouldn't have the manpower to use them effectively. Just surrender now, because you have no chance."

The purpose of this statement is to:

Undermine confidence: Make the people doubt their ability to defend themselves.
Intimidate: Showcase the overwhelming power of Assyria.
Encourage surrender: Make surrender seem like the only rational option.
Discredit Hezekiah: Weaken the king's authority by portraying his kingdom as weak and helpless.

This verse is a powerful example of propaganda and psychological manipulation used by the Assyrians to conquer and control other nations.