'Publication citations' refers to the number of times that a publication has been cited by other publications in the Dimensions database. (Publications here includes more than just research articles, but also books/chapters, monographs, preprints, and conference proceedings).
As a standard measure of the impact a paper has, citations are the most commonly used indicator of the acceptance/importance of published work within the research community. This is despite the fact that the number of times cited does not include any indication of the nature of a citation (favourable, or merely observational), or whether it has been cited in a primary research or review paper. The number of times cited is also closely related to the length of time since the paper was originally published, as even papers which represent a breakthrough in their field will have a low citation count if they have been published recently.
Aggregated indicators based on citations - available in the Analytical Views
'Citations per publication' is an aggregated citation-based indicator; it is calculated by dividing the total number of citations by the total number of publications.
'Cited/Not Cited' is the percentage of publications that have been cited one or more times vs. publications which haven't been cited yet.
'Citations (Mean)' is the mean average citation number for a given group of publications being analysed. (This is the arithmetic mean, meaning that it is greatly influenced by outliers. If you are looking for the true "middle" point in a group, the median average citations (below) may be more suited.)
'Citations (Median)' is the median average citation number for a given group of publications being analysed.