Site is the name of the site or area the article relates to.
O&D items in the newsletter are numbered sequentially. Some of the Notes make reference to those numbers (e.g. further info following report in 14).
N/L relates to the newsletter number in which the article appeared. (Not every Newsletter includes an O&D article). This is a clickable link to the relevant newsletter PDF, you need to browse that to find the O&D section.
GridDesc is the OS NGR (National Grid Reference) as shown in the original item, sometimes that is a group of NGRs e.g. representing a group of features e.g. "centred SK 188 758"
Notes - the item headline, for further detail see the article
OS NGR is a single grid reference to the approximate location of the item discussed (as distinct from GridDesc). It is a clickable link to OS Explorer 1:50k mapping.
Coordinates is the latitude & longitude of the item and is a clickable link to a Google map ("Satellite" view).