What does it mean when it is stated that you have 8 shared segments with a particular individual?
"8 shared segments" means there are 8 separate sequences of over a "significance" threshold (this is often 7 centimorgans or cM) within which you match the other individual. To be included in one of these matching segments, each base pair must be at least a half match.
Below is an illustration of the 2 shared sequences I have with my uncle on the first chromosome pair. The illustration (from gedmatch.com) shows base pairs with a half-match in yellow, those with a full match in green, and those with no match in red.
Broadly speaking, the more sequences of above a certain threshold, the more significant the match. This threshold depends on the context (for example, in endogamous populations where there has historically been a closed gene pool, a match might have to be longer (e.g. more centimorgans) in order to be considered significant. In my experience, a sequence of at least 20cM tends to indicate a match that has the potential to be traced.