Link velocity literally means "link speed" and tells you how many incoming links you have and how fast this number of links is growing. Link velocity tells you how fast you are currently generating new external backlinks.
To improve your link velocity, qualitative link building is more important than quantitative link building. The rapid accumulation of non-qualitative links can be counterproductive and detrimental to your search engine optimisation (also known as SEO). For example, if you generate a hundred backlinks in one day, Google sees this as unnatural link building and spam.
By slowly building up quality backlinks and reliable referring domains, you can improve your position in the search results.