what pivot tables are?

Ran into problem with pivot tables. There's existing workbook A that uses data from workbook B with pivot tables. Somehow data for certain date there doesn't show up. Otherwise works fine though.

Trying to find answer for that but I have little idea about what pivot tables are and how they work. Could somebody give quick overview of them and how to see precisely what it retrieves and where? Bit of hard to debug when macro assigned to button just refresh pivot table and that's it.