Long documents often contain references of the sort “see chapter xx on page yy” or “see illustration nn on page mm”. These placeholders get automatically substituted by the actual numbers. You wouldn’t want to write literally “see chapter 12 on page 36”, because these numbers might soon become obsolete as you move chapters around or include further illustrations etc.
To reference a chapter go to Insert › Cross-reference › Cross-references and choose Headings in the top left hand column. In the right hand column pick out the chapter heading you would like to refer to. In the bottom left hand column select whether to refer to the number of the page the heading is situated on or the number of the heading itself (providing you have numbered headings) or even both, then confirm Insert and Close. In similar fashion you can point to tables or illustrations (providing they have numbered captions).
All cross-references are listed in the Navigator (F5).