What is a Radius clause in a separation agreement? My spouse wants us to get a radius clause.

A radius clause is basically an agreement between the two parties not to relocate outside a specified radius. This kind of provision in a separation agreement protects both parties from the sudden relocation by the other and the diffiuculties that would pose for the children who need to have a relationship with both parents.
The court cannot necessarily enforce radius clauses. But the court can factor that into its decisions if the parties go back to court on the grounds that one of them violated the radius clause in the separation agreement. If one party seeks a modification of the visitation agreement on that basis, he or she would probably get it, given the violation of the radius clause.
But it’s not like the court can tell a parent, “you can’t relocate.” That would be like an involuntary arrest or something and would be unconstitutional.