Reactor grade material is usable in most nuclear power plants.
Weapons grade material is required for nuclear weapons.
For uranium the difference between reactor grade and weapons grade is the level of enrichment: less than 20% uranium-235 is reactor grade, greater than 20% uranium-235 (greater than 90% is prefered) is weapons grade.
For plutonium the difference between reactor grade and weapons grade is the level of contamination with plutonium-241: any amount of plutonium-241 is OK for reactor grade, only low levels of plutonium-241 are acceptable in weapons grade as its spontaneous fission rate can cause the bomb to fizzle.