Numbers 23:20 is part of a longer narrative in the Book of Numbers where Balak, the king of Moab, hires Balaam, a non-Israelite diviner, to curse the Israelites who are encamped nearby and pose a threat. However, instead of cursing them, Balaam is compelled by God to bless them.

Let's break down the verse to understand its meaning:

"Behold, I have received a command to bless." This highlights that Balaam's intention was something other than blessing. He received a specific command from God to bless Israel. This implies that Balaam is acting against his own will or original intentions. He is a reluctant instrument of God's will.

"He has blessed..." This refers to God's prior act of blessing Israel. This is an established, divinely ordained blessing that Israel already possesses.

"...and I can’t reverse it." This is the key point of the verse. Because God has already blessed Israel, and because God has commanded Balaam to bless them further, Balaam cannot curse them. The blessing is irreversible. This highlights God's power and sovereignty over the situation. No matter what Balak wants or what Balaam attempts to do, God's plan and blessing for Israel will prevail.

In essence, the verse signifies the following:

God's Sovereignty: God is in control. He can use even those who are not his followers (like Balaam) to accomplish his purposes.
The Irrevocable Nature of God's Blessing: God's blessing is powerful and enduring. It cannot be undone by any human or supernatural force.
Protection for Israel: This verse provides assurance to the Israelites that they are protected by God and that no curse can harm them.
Frustration of Evil Intentions: The verse shows how God can thwart the plans of those who seek to harm his people. Balak's scheme to weaken Israel through curses is ultimately unsuccessful.

The verse emphasizes God's faithfulness to his covenant with Israel and his power to protect them, despite opposition. It serves as a powerful declaration of God's unwavering commitment to his promises.