A large document requires a lot of input. Rather than putting the whole input in a single large file, it's more efficient to split it into several smaller ones. Regardless of how many separate files you use, there is one that is the root file; it is the one whose name you type when you run LaTeX.
Dlouhe dokumenty potrebuji obsahly vstup. Je lepsi tento vstup rozdelit do vice casti, nez vytvorit jeden mamuti soubor. Bezrozdilu kolik vstupnich souborku pouzijete, musite mit porad jeden - "korenovy", coz je ten, s jehoz jmenem volate LaTeX.