Randomization is a process used in scientific experiments to assign study participants to a treatment condition randomly. It may involve a simple method, like a coin toss or drawing numbers blindly from a basket, or it may involve more elaborate methods that involve a computer program.
Participants that have been randomized to the intervention condition receive the treatment that is being evaluated. Participants that have been randomized to the control condition receive a different intervention.
Because the assignment to the two different treatment conditions is done randomly, the two groups are expected to be statistically equivalent at the start of the study. Any differences at the end of the study can be said to be due to the intervention itself.