Randomisation is a method of allocating or selecting without using any system. It is purely random. In clinical trials, participants are generally allocated to different arms of the trial (for example, to receive the study medicine or the placebo) randomly. This is a key part of the randomised controlled trial (RCT).
Randomisation in clinical trials means that each participant has an equal chance of being in any of the arms of the trial. It is an important method to reduce the risk of bias in the outcomes of the trial.