  The advent of the sixth-generation (6G) networks presents another round of
revolution for the mobile communication landscape, promising an immersive
experience, robust reliability, minimal latency, extreme connectivity,
ubiquitous coverage, and capabilities beyond communication, including
intelligence and sensing. To achieve these ambitious goals, it is apparent that
6G networks need to incorporate the state-of-the-art technologies. One of the
technologies that has garnered rising interest is fluid antenna system (FAS)
which represents any software-controllable fluidic, conductive, or dielectric
structure capable of dynamically changing its shape and position to reconfigure
essential radio-frequency (RF) characteristics. Compared to traditional antenna
systems (TASs) with fixed-position radiating elements, the core idea of FAS
revolves around the unique flexibility of reconfiguring the radiating elements
within a given space. One recent driver of FAS is the recognition of its
position-flexibility as a new degree of freedom (dof) to harness diversity and
multiplexing gains. In this paper, we provide a comprehensive tutorial,
covering channel modeling, signal processing and estimation methods,
information-theoretic insights, new multiple access techniques, and hardware
designs. Moreover, we delineate the challenges of FAS and explore the potential
of using FAS to improve the performance of other contemporary technologies. By
providing insights and guidance, this tutorial paper serves to inspire
researchers to explore new horizons and fully unleash the potential of FAS.
