Mosaic was a subtheme of the LEGO Basic theme, which began in the 1970s, was designed to be a more general, non-thematic series of sets. Unlike the themed sets that revolve around specific stories, characters, or concepts (e.g., space, city, or pirates), the Basic theme provided a collection of standard LEGO bricks and pieces, often in a variety of colors. The goal was to encourage free building and creativity without the confines of a specific theme or narrative.
The Basic sets often came with simple instruction booklets that showcased a variety of builds possible with the given pieces, but they were fundamentally meant to inspire open-ended play and creativity. The Basic theme persisted in various forms for many years, catering to builders who just wanted a collection of bricks to build whatever they imagined.