What is a net dipole?
Dipole moments occur when there is a separation of charge. This occurs due to an atoms' electronegativity - where one atom has the ability to attract electrons towards it (In other words, electrons wants to spend other time around it) giving it a negative charge and the other a positive charge.
Net dipole operates on the same idea - but it focuses on the direction and magnitude (vector) of the dipole moment. Focusing on the image above (a), carbon dioxide - oxygen being more electronegative than carbon pulls electrons towards it as demonstrated through the arrow. As the molecule is linear, both the bond dipoles cancel each other out (i.e. the vector addition of the dipoles equals zero) and the overall molecule has a zero net dipole moment.
However, for (b) as both vectors are pointing in the same direction, it has a net dipole charge.