Cannabidiol (CBD) is an abundant non-psychoactive phytocannabinoid in cannabis extracts which has high affinity on a series of receptors, including Type 1 cannabinoid receptor (CB1), Type 2 cannabinoid receptor (CB2), GPR55, transient receptor potential vanilloid (TRPV) and peroxisome proliferator-activated receptor gamma (PPARγ). By modulating the activities of these receptors, CBD exhibits multiple therapeutic effects, including neuroprotective, antiepileptic, anxiolytic, antipsychotic, anti-inflammatory, analgesic and anticancer properties. CBD could also be applied to treat or prevent COVID-19 and its complications. Here, we provide a narrative review of CBD's applications in human diseases: from mechanism of action to clinical trials.