The concentration of the wine is referring to the ripeness level of fruit (the grapes) that made the wine. If a wine has a high concentration of fruit then it means that the grapes used were very ripe and are showcasing a wide, or full, aromatic spectrum. The fruit concentration of a wine can be perceived, or thought to be perceived, as giving the wine sweetness – this is just the quality of the fruit giving richness to the wine. For example a Barossa Valley Shiraz will often seem sweet on the palate but in reality it has just got really ripe fruit and a very high fruit concentration.
The fruit concentration of a wine is one of the leading marks of quality. Though just because a wine has a high fruit concentration does not mean it is balanced, so a wine that has a high fruit concentration is only of top quality assuming it has the acidity or tannins to balance it out.