what is zero work?

Hint: Work can be understood in context of two meanings- scientific notions of work and physical notion of work.

Complete step by step solution:
When we talk about work we generally think about some kind of physical movement being taking place or some laborious task needs to be done. In Physics, work is defined as the dot product of force and displacement.
\[W=\overrightarrow{F}.\overrightarrow{d}\]
Here W refers to work
F refers to force
D refers to displacement

We should keep in mind that force and displacement both are vector quantities and here is the dot product of force and displacement. The dot product of two vectors gives us the scalar result.
\[W=\overrightarrow{F}.\overrightarrow{d}=Fd\cos \theta \]

Now if the angle between force and displacement is \[90{}^\circ \]then we know the value of cos\[90{}^\circ \]is zero.
\[\begin{align}
  & W=\overrightarrow{F}.\overrightarrow{d}=Fd\cos \theta =Fd\cos 90{}^\circ =0 \\
 & \\
\end{align}\]

Hence in this case when the angle between the force vector and the displacement vector is \[90{}^\circ \]then the work done is zero.

Let us consider an example of zero work. When we hold an object and walk, the force acts in a downward direction whereas displacement acts in the forward direction. In the case the only force that is acting on the object is the force of gravity which is acting downwards & the displacement is in the forward direction. So the angle between the force vector & displacement vector is \[90{}^\circ \] hence the resultant work done is zero.

Note: Although energy is being used but scientifically work done is zero. So it depends upon our criteria of looking at the process.