Search volume is a figure that is provided by Google.

It specifies the average number of people who search in Google for a given 'keyword' every month.
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So, let's say 'SEO' had a search volume of 100,000. This means that (on average) 100,000 people search the term 'SEO' in Google every month.

The important word above is 'average'. What Google do is take the yearly figure and then divide it by 12 to get the monthly number.

So it doesn't show you seasonal trends, so be mindful of that (e.g. some months may have way more people searching that term that others, but Google only shows an average over the whole year).