In eukaryotes, which you are probably studying, glycolysis occurs in the cytoplasm of the cell, the Kreb's cycle and the PDH complex occur in the mitochondrial matrix, and oxidative phosphorylation is on the inner mitochondrial membrane. NADH and FADH2
are already in the mitochondria where they need to be for oxidative phosphorylation, but glycolytic NADH is all the way outside in the cytoplasm. In order for the glycolytic NADH to be where it eventually needs to be (in the mitochondrial matrix), it must be transported by a pathway called the glycerol phosphate shuttle. Due to this, there are only 1.5 ATP produced per NADH from the cytoplasm whereas Kreb's cycle NADH produces the full 2.5 ATP per NADH.
(I don't know if you need to know about prokaryotes, but for completeness I'll explain. Prokaryotic glycolysis occurs in the cytoplasm, but because there are no mitochondria in prokaryotes, oxidative phosphorylation occurs at the cell membrane instead. All NADH produced are in the correct locations and do not need to be transported, therefore all NADH produce the full 2.5 ATP/ NADH. This explains why eukaryotes produce a net total of 30 ATP per glucose, while prokaryotes can produce a net total of 32.