Numerous factors contribute to a noisy video generally caused by light-based issues such as insufficient light in the shot, incorrect low-light lens, extremely high ISO values, heat or interference. However, it must be noted that there is no better fix for grainy or noisy footage than correctly taking the time to shoot it out in the field.
However, if you already have a clip that has to be denoised, you can get that done either with the help of Neat Video or using free effects that are already built into Premiere Pro. Learn more.