Nebari is a Japanese word. You may hear it when describing the lowest part of the trunk and the roots visible on the surface of your bonsai.
It is sometimes referred to as 'buttressing.' Why is it valuable? When roots are flared or spreading they give the effect of a more aged tree.
Based upon the bottom section alone of these bonsai, which of the tree trunks below looks older and more mature?
It's possible the tree to the left is older than the one on the right ... however, the trunks tell us something different.
One of the goals of a good bonsai is to create the appearance of age. Thin trunks with little to no side roots do not look old.
Some species have naturally spreading roots. It's much easier to expose roots on these species in order to add beauty and perceived age to the tree.
A few such plants that come to mind immediately are Azalea, Trident Maple and the tropical Ficus species.
Often Ficus make fantastic shohin because of this attribute.
Another asset of such rootage is, this type of tree often allows the roots to spread while growing in shallow pots. This shohin Ficus by Suthin is a perfect example.
Spreading roots are not always obvious. When designing your own bonsai tree from nursery stock, one of the first bonsai basics is to look for such treasures while determining the front of your tree.
Lower the soil level, expose some of the lower trunk. It's usually not necessary to dig very deep. "Dig" or scrape slowly and carefully as to not damage the trunk or roots. A chopstick is often used for this process.
Combine the exposed roots, the shape of the tree and placement of branches to discover the front of your tree.
Sometimes folks style their tree strictly based on where the branches are ... only to discover later that if they had exposed some of the rootage, they may have had an entirely different (and perhaps better) bonsai.
For upright trees, evenly distributed roots give a sense of balance.
Good buttressing can be different for different styles. For a slant style it may be a bit one-sided to show that the tree will not fall over.
(Peter Tea worked on this tree while he was in Japan.)
Strong roots are exposed on the right to show the tree is stable and is secure in the ground.
If your rootage seems lop-sided, but you have a great tree otherwise, try one of Harry Harrington's techniques for correcting this problem.
Not all "exposed roots" are considered nebari.
The almost ugly, high standing roots on this pine by Walter Pall are called neagari.
These roots and the tree itself seem to go together for a unique looking bonsai tree.
(In some cases roots like this look almost like legs.)
Read more about this and see pictures on the Neagari Page.