The volume column displays the exact match search volume for a month. Exact match means searches for exactly that phrase. This data comes from Google, and it is only an estimate, so don't get discouraged if it is low - it can be quite inaccurate for long tail keywords.
Google group volumes for similar keywords together. For example, plurals. The keywords "house" and "houses" will have the same search volume reported by Google. It may be the case that the keyword "house" actually contributes half of the total volume and "houses" contributes the other half, so it is important to be aware that Google search volumes can actually over estimate the volume for keywords that have plural versions or other minor variations.