The phrase "are binding on" means that the Security Council resolutions are legally enforceable. This means that all UN member states are required to comply with the resolutions, even if they do not agree with them.
The phrase "are binding on" is used in legal contexts to describe the legal force of a document or agreement. In the case of Security Council resolutions, the legal force comes from the UN Charter, which states that all member states are obligated to comply with the decisions of the Security Council.
The phrase "are binding on" can also be used in a more general sense to describe the obligation to comply with a rule or regulation. For example, you might say that "all drivers are bound by the speed limit."
In the sentence "Security Council resolutions are binding on all countries in the UN," the phrase "are binding on" means that all UN member states are legally obligated to comply with the resolutions of the Security Council. This means that if a Security Council resolution orders a country to take a certain action, the country is legally required to do so.