You can label a section (or subsubsection, or, more rarely, just a random piece of text) in your document. Once you do so, you can refer to this section in other parts of the document, using cross-references. You can refer either to the section's number, or to the page that the section appears on. As with sections and footnotes, LYX worries about the cross-references for you. Automatic labels and cross-references are one of the best advantages of LYX (and LATEX) over conventional word processors.