Galactic cosmic rays (GCRs) are continuously propagating high-energy charged particles originating outside the Solar System, whose intensity near Earth is significantly modulated by solar activity. One of the most prominent manifestations of this modulation is the transient reduction in GCR intensity known as a Forbush decrease (FD), which typically occurs in association with coronal mass ejections (CMEs), interplanetary shocks, and solar flare activity. This paper provides a comprehensive review of the primary physical mechanisms responsible for the suppression of cosmic ray flux during FD events. These mechanisms include magnetic shielding caused by enhanced interplanetary magnetic fields embedded within CMEs, shock-driven disturbances, and increased turbulence in the heliosphere. Observational data obtained from ground-based neutron monitor networks and space-borne instruments are analyzed to illustrate typical FD temporal profiles, recovery phases, and amplitude variations. The observed decreases in cosmic ray intensity generally range from approximately 3% for weak events to more than 20% for strong solar disturbances. Four illustrative figures are presented, showing characteristic FD events, schematic diagrams of CME–Earth interactions, shock structures, and large-scale heliospheric magnetic field configurations. The results emphasize the dominant role of strengthened interplanetary magnetic fields in deflecting and scattering low-energy GCRs, thereby reducing their access to the inner heliosphere. This study contributes to a deeper understanding of cosmic ray modulation processes and their relevance to space weather forecasting and heliophysical research.