Search Volume is a metric to understand demand for a given Keyword. It is an estimate of monthly searches for a term. The higher the metric the more visits you can expect when ranking high in SERPs for a given Keyword. However, that keyword may also very likely be more competitive. Tools such as Ahrefs give you an indicator of how competitive the keyword is. On the other side of the spectrum, if you target Keywords with no search volume, you are less likely to get visitors to your site.