This is part of my Physics notes on the concept of shearing. Shearing is when an object is warped like you see in the diagram. For graphic designers and Photoshop users, "Skewing" is probably a more familiar term. That's basically it.
This diagram in particular is for fluids because, normally, there wouldn't be "shear rate" unless it's something undergoes constant shearing... like a flowing but viscous or "thick" liquid.

One might think.. okay... why would a liquid shear? Well, liquids normally have adhesive properties and more viscous liquids are generally more adhesive.

But even a non-viscous liquid like water has adhesive properties. You can see this when you notice that water/rain sticks to glass windows and other solids. You can try to shake it off or wipe it off but you'll have a hard time getting it dry unless you have the water evaporate.

Because of this adhesion, a liquid can undergo shearing like a solid. Shearing in solids happens when different sides of a solid body is pulled in different directions. This has been known to happen when the earth's tectonic plates move around during an earthquake, causing man-made structures to bend and break from the forces going in different directions.

The same thing happens with a viscous liquid like in the case of a tilted jar of honey. The honey around the edges touching the glass jar tends to stick to the jar while the rest of the honey tends to flow due to gravity. There, you have at least three opposing forces: (1)adhesive forces keeping the honey stuck to the jar, (2)the force of gravity pulling the honey downward, and (3) the intermolecular forces keeping the mass of honey intact. Since all these forces are usually present for longer periods of time, there's a point to measuring the shear rate (shear strain per unit of time).

But carefree square generally doesn't care about such things. Look at how happy she is.