I did a few quick searches- it's talking about a 'visual search', which is really more related to psychology than to computer science.
The idea is that the brain filters objects based on criteria- for example, if the task is to spot a green square in a grid of yellow triangles, the brain might look for green objects. However, if the task is to find a green square in a grid of green triangles and yellow squares, it isn't so simple. The brain might have to look for green objects first, then look through those for a square. Since it has to do more searches, and search more objects, the second task would tend to take longer.
There are apparently a few different theories on how it actually works, though, so I'd recommend searching "visual search" or "psychology visual search" online and finding a few good, understandable sites.
Hope this helped, and if you have any other questions, just ask!
Nature uses only the longest threads to weave her patterns, so that each small piece of her fabric reveals the organization of the entire tapestry.