Essentially it means the speed at which you build links. Google measures the speed at which backlinks are created as a measure of unnatural link building. This, in some instance indicates "spamming" but not always, as viral content normally has a high backlink velocity which Google needs to take account of.
Link velocity is the rate at which others are linking back to your site. ... Building quality backlinks should be the main priority, not achieving a high link velocity. However, link velocity is one of the SEO metrics that you should track.
Link Velocity is the rate at which others are linking back to your site. It is a way of measuring the rate at which a website’s backlink profile grows over time.
It is also known as the rate at which you add incoming links to your website. Link velocity is principally emphasized on Off page because it includes the measurement of backlinks associated with any website.
Its the rate of getting or earning links in your link profile. But here comes the question should we be concerned about it? The answer is NO. As long as they are relevant.
Its the rate at which you are building your backlink profile, it is one of the ranking factors but one should not be concerned about it. It is one of the google's signal to work not for the website owner. Main focus be building the content, earning natural backlinks through good trustworthy websites. Even if the rate of building the links is rapid or slow as long as you are honest with the content, everything works fine.