Search volume is a metric that tells you how many people search for a particular query. You can find the search volume for particular queries by using tools like Spyfu and Google Trends. Optimising your content for queries with a high search volume means significantly more impressions – if you manage to rank in the top results. The catch of optimising for queries with a high search volume is that there’s often a lot more competition, making it harder to rank highly.
See also: Competitor keyword analysis, Keyword competition, Keyword research
Further reading: What is Google Trends, and Can I Use it to Find Keywords?