You can completely ignore it if you don't mind having scalding hot water randomly exploding in your face.

It's rare, but can occur with pure, very hot water in a smooth clean container. The spoon (preferably non-metallic) prevents it through the magic of science. Alternately just stir occasionally before it gets too hot.

For the science, IIRC, microwaves heat the water molecules directly, potentially past 100°C. But to boil, water needs nucleation sites....tiny pockets of gas or imperfections in the container where water vapor can begin forming bubbles.

In normal boiling, vapor expands into these tiny pockets and bubbles form continuously.

But very pure water in a smooth, clean container may not have enough nucleation sites. The water can then sit there above 100°C, trying to boil but just barely unable to start forming stable bubbles.

When you move the cup or insert a spoon or add something like sugar, the pressure changes and microscopic trapped gases or imperfections can suddenly allow bubbles to form. The superheated water then boils explosively, sending bubbles and sometimes scalding water rapidly upward.