Spatial skills are cognitive skills associated with understanding space and spatial operations. Spatial skills involve tasks like mental rotation of 3-dimensional objects, understanding how a flat pattern could fold up into a 3-dimensional shape and being able to visualise a cross-section of a complex structure.
Parts of our brain are well developed to understand and store spatial information, so that early humans could keep track of the world around them. However, it turns out that these brain structures can be used to understand much more than just spatial information, but in fact any non-verbal information.
Spatial skills appear to be closely related to these brain structures, which explains why they are so often related to success in many aspects of learning.
Spatial skills are malleable and can be trained, which has been demonstrated in many different contexts. Many different activities can train spatial skills, like using construction toys or blocks, map reading and playing video games. But it is not common to see spatial skills developed in typical school curricula. We broadly recognise the importance of maths and literacy, but not spatial skills.
Since spatial skills aren't developed formally in school, they develop irregularly in school-aged children. Children who happen to have the right experiences and opportunities, which are often gendered and can be prohibitively expensive, are the lucky ones, who will be well set up for many fields of later study. But children who miss out on these experiences and don't get a chance to develop their spatial skills might end up struggling in some classes later in school.