NMM is short for Nexus Mod Manager.
Download all files labelled REQUIRED. There will typically be two files to download: "Better Cities" and "Better Cities Primary Resources"
At the end you should have five BSAs from Better Cities, one ESM, an ESP named "Better Cities .esp" and various other ESPs depending on the choices you made. If you have less than five BSAs, then you installed the mod wrong.
There may be an additional archive to download containing a hotfix or updated files to fix any bugs or missing files from the main archives. This may contain additional BSAs.
If you want more information about what a specific ESP in the BC archive does, then read What Does Each File Do.
If updating from an older version, please ensure that you uninstall the old version before installing the new version.
Simply place the two downloaded archives into the "Nexus Mod Manager\Oblivion\Mods\" folder, then select the archives from the list in Nexus Mod Manager and click the green tick to install them. Answer each query.
You must have 5 BSA installed.
Adjust your load order so that all Better Cities ESPs are near the end of your load order. All Replacer ESPs should be BEFORE the Better Cities ESPs. All Better Cities Patches should be AFTER the Better Cities ESPs, starting with the FPS patches, then the compatibility patches. Highly recommend using BOSS (Better Oblivion Sorting Software) for this.