This number is an estimation of how much paid search traffic your target website, subfolder or URL gets each month from search ads.
In this case, Hubspot is estimated to receive ~ 1 click per month from their ad via the paid keyword "best crm systems"
How it’s calculated:
We find all the keywords for which your target shows an ad in the top 100 search results.
We estimate the search traffic your target gets from each of those keywords based on the ad's position, monthly search volume, and our estimated Click-through rate (CTR) for that position.
We sum up the traffic estimations of each keyword
Traffic coming from display ads, video ads, and other non-search ad types are not captured.
Limitations with paid traffic estimations:
It is calculated using estimated values such as search volume and CTR, and thus has similar sources of discrepancy that organic traffic has.
Even if a website is paying for their ad to show up to a certain keyword, the ad usually will not show up every time that keyword is searched. How often an ad shows up and the SERP position it takes depends a lot of things:
advertiser's bid amount
location
time of day
As such, its possible not all paid keywords will be shown in the paid search reports.