Your mother is right; you should go take a walk.
Environmental psychology research has identified a slew of reasons why your mother's suggestion is a good one - and all the psychological benefits of a walk complement the physical pluses of burning off a few calories, toning your muscles, and absorbing some vitamin D (on outdoor walks).
When you're walking at your own pace, your cognitive performance improves - more specifically your working memory is better than it is when you're sitting still. Working memory is important for reasoning and learning, for example. If you're hurrying or walking slower than you want to be traveling, you won't get the memory boost.
If you're walking outside, there are additional benefits of perambulating around. Being in daylight helps keep our circadian rhythms appropriately aligned with the place that we live - which is great, because when they're not, we experience stress. Being in daylight also improves our mood and general feelings of well-being. Positive moods have been linked to broader and more creative thinking.
A walk in a park with lots of grassy areas and trees might be the very best of all. Being in nature, and even looking at nature, helps people who are cognitively exhausted, from long periods of knowledge work, for example, restock their mental energy. Walking through a completely wild, jungle-like setting won't do the trick, there's too much potential for danger to be lurking around the corner there. We feel most comfortable in spaces that combine grassy areas and trees.
Even a short time outside has benefits. Spending as little as five minutes in a nature setting if you're engaged in some sort of physical activity results in large improvements in self-esteem and mood. Longer periods in nature generate additional benefits, although the per-minute return decreases after the high values of those initial five minutes. The greatest psychological benefits accrue to people walking through a natural setting.
So, lace up those sneakers and take a spin around the block - it'll be good for your mind and your body.
As always, references for the scientific studies on which the material in this article is based are available from Dr. Augustin at firstname.lastname@example.org.