When you call a person a minimalist, you're describing their interest in keeping things very simple. A minimalist prefers the minimal amount or degree of something.
In art history, the minimalists were artists whose work involved extremely simple gestures and ideas. The art they created is also referred to as minimalist. A white canvas with one black line might be a typical minimalist painting. But anyone who likes things very simple could be called a minimalist. A president who takes a hands-off approach to policy, a composer who only plays one note, a designer who puts only a bed in a bedroom: they're all minimalists.