When a mod you installed requires you to install another mod for it to work, the other mod, in this case, is a required dependency. However, some mods may require you to install other mods for them to work and can also make suggestions for you to install other mods for them to extend their functionality. Many mods use Factorio's base mod as their only dependency which you do not have to install separately. These are installed the same way as mods downloaded from the portal. They will be downloaded as ZIP files, either as forum 'attachments', or with a link to a hosting site. There may be experimental mods or updates here that aren't available on the main portal. Mod authors maintain threads in the official mod subforum to support their work. Verify a successful installation by viewing the 'Mods' list through the main menu, in-game. Mods come as ZIP files, installed by copying (not unzipping) into the 'mods' directory in the user data directory.
The mod portal (website) is the center of mod hosting, where authors upload mods, and you can find previous versions, and discussions. It combines downloading & installing, checking installed mods for updates, and enabling/disabling installed mods. The 'Mods' section of the main menu is the best way to get mods. You can download the mods from the following places:
