The Page Numbering Mode is used to define when automated page numbering is reset to '1' (or A, or i, etc. depending on the Page Numbering style set for that Section).
Continue from Previous Section means that the page numbering continues from the last page number of the previous Section, regardless of which Section type preceded the defined Section. For example, you might have a document that contains a Back Matter Section that needs to continue its page numbering from the last Chapter of the document.
Restart at First Section means that page numbering is reset to '1' for the first occurrence in the publication of the defined Section. Second and subsequent occurrences of that same Section will have their page numbering continue from the last page number of the previous Section. This setting allows the first Chapter to start on page 1, and the second Chapter to continue numbering from the end of the first Chapter, for example.
Restart at Every Section means that the page numbering is reset to '1' each time the defined Section appears in the publication.