In a randomized controlled trial, chance decides who receives the thing being tested and who receives a comparison, such as a placebo, a sham, or usual care. Because chance does the sorting, the groups tend to be alike in everything except what is being tested, so a difference in how they fare can be credited to it.
The idea grew over centuries. In 1784, a royal commission in Paris used blindfolds and deception to test whether “animal magnetism” existed (Franklin et al., 1784/2002). In 1948, the Medical Research Council’s trial of streptomycin for tuberculosis kept its random assignments in sealed envelopes, so no investigator knew which treatment the next patient would get (Medical Research Council, 1948). That care against allocation bias, so that like was compared with like, is why the trial holds its place in the history of clinical trials (Chalmers, 2010).
How to read one
Ask what the comparison was, how many people took part, whether the people measuring outcomes knew who received what, and how large the difference was, not only whether it was statistically significant.
References
Chalmers, I. (2010). Why the 1948 MRC trial of streptomycin used treatment allocation based on random numbers. The James Lind Library. https://www.jameslindlibrary.org/articles/why-the-1948-mrc-trial-of-streptomycin-used-treatment-allocation-based-on-random-numbers/
Franklin, B., Majault, Le Roy, Sallin, Bailly, J.-S., D’Arcet, De Bory, Guillotin, J.-I., & Lavoisier, A. (2002). Report of the commissioners charged by the king with the examination of animal magnetism. International Journal of Clinical and Experimental Hypnosis, 50(4), 332–363. https://doi.org/10.1080/00207140208410109 (Original work published 1784)
Medical Research Council. (1948). Streptomycin treatment of pulmonary tuberculosis: A Medical Research Council investigation. BMJ, 2(4582), 769–782. https://doi.org/10.1136/bmj.2.4582.769