In certain cooler climate wine regions, the grapes may not ripen sufficiently to produce a must high enough in sugar. In such cases, you are permitted to chaptalise, i.e. add beet or cane sugar to enrich the unfermented grape must. This will lead to an increase in alcohol content after fermentation. This practice is also known as must enrichment.
It is strictly forbidden in many countries and regions. However, it is generally allowed in some regions where the grapes tend to have low sugar content, for example some regions in Germany and northern France.