My understanding is that a relative brace (for a column) would be something like a diaphragm that will help to brace all columns at that level. A nodal brace is more along the lines of a wind girt - since it may only be bracing (2) column and not all of the columns.
Sortof StructuralEIT. Fig. C-A-6.2 on p16.1-422 does a pretty good job of illustrating this concept for both relative and nodal bracing on columns and beams.
A relative brace is a brace between two objects that moves along with the objects - they are objects being braced are being braced RELATIVE to one another - but the entire system can still be swaying. For instance the diagonals in a braced frame are relative - they brace the columns however the entire frame can still sway. Think of it as INTERNAL bracing to the system. These are the most common types of braces in building construction.
A nodal brace braces an object from an exterior "rigid" point. Think of a big concrete shear wall with beams over to a moment frame. The beams brace the columns at descrete points, the columns are not braced relative to one another.
