Ubuntu Minimal is a small disk image for installing Ubuntu. It will get packages from the internet during installation. Not to be confused with the new "Minimal installation" option in the Ubuntu 18.04 Desktop installer (see tag "minimal-install").
The minimal iso image will download packages from online archives at installation time instead of providing them on the install media itself. Downloading packages at install time reduces the size of the iso image to approximately ~40MB depending on architecture (see below), as well as providing only the packages needed for installation. The download time savings achieved by using a mini iso can be significant, as only current packages are downloaded, so there is no need to upgrade packages immediately after installation. The mini iso uses a text-based installer, making the image as compact as possible.
Source: Ubuntu Wiki - Installation/MinimalCD
Not to be confused with the new "Minimal installation" option in the Ubuntu 18.04 Desktop installer - see tag minimal-install