As the pace of global 5G network deployments accelerates, now is the moment for the cellular industry to realize 6G cellular communication. In this article, modular massive multiple-in-put multiple-output (mmMIMO) is presented as one candidate technology for 6G to improve the spectral efficiency in low-frequency bands. The 5G New Radio pushed the boundary of the cellular system's operating frequency to high-frequen-cy bands, and this trend will continue in the 6G era. However, the technical advances in 5G for low-frequency bands fall short, although low-frequency bands are crucial in serving a large number of users in a wide coverage area. Although it would be ideal if massive MIMO could be utilized in low-frequency bands, it is less practical due to a large antenna form factor size. mmMIMO is a technology to distribute a large active antenna array with smaller standardized antenna modules, just like Lego-type building blocks. Through this, the benefits of massive MIMO can be achieved in low-frequency bands (e.g., sub-1 GHz), unconstrained by spatial limitations. In this article, the concept of mmMIMO, its applicability, and needed research efforts to standardize the technology for 6G are discussed. In addition, through the demonstration of a proof-of-concept system, it is shown that the technology can be within reach at the time of 6G commercialization around 2030. Lastly, the performance gain of mmMIMO is evidenced by system-level simulation.