In terms of author metrics, two main statistics are used to determine impact: total citation count and h-index.
As you read under the Article Impact and Metrics tab, citation count is an article level metric. But it is common to use citation count as an author level metric as well. Authors will sum up the number of citations from all of their publications to create a "total citation count" for themselves. Web of Science can be used to calculate a total number of citation, with and without self-citations.
H-index is a little different. H-index tries to tie together both the productivity (or volume) of works published, in addition factor in the impact of the works published. H-index formulas vary and the h-index formula used by Web of Science is actually trademarked and patented, so you will only find variations of h-index in Scopus and Google Scholar.