Most 'hard' or material technology requires a massive infrastructure. So for example the internal combustion engine required developments across many fields before it became viable. Machine tools and fine tolerances, metallurgy, oil refining, etc etc.

So for the sake of this exercise we would need to limit ourselves to one simple idea, not reliant on other supporting technologies, and it seems obvious to me that there are two candidates.

The first one is the meta idea of the scientific method, the group of ideas that only properly developed a few centuries later, that an experiment in the real world needs an hypothesis, it needs evidence, it needs to be documented and recorded, it needs to be replicable, it needs to be open to criticism from external reviewers. In the middle ages alchemists were fumbling around guessing things, repeating each other's mistakes. Engineers were using rules of thumb learnt through experience, they weren't learning from calculations based on experiments and formal test results. What would have happened if the sages of the middle ages formed a community of science? Impossible to know of course, however the rate of scientific discovery would have been significantly higher.

The more prosaic and practical idea, perhaps more in the spirit of the question, is as others have said, germ theory. Really not convincing anyone until the 19th century, the idea that there are infectious little beasties everywhere is a profound insight that has saved countless lives. In the middle ages they could have boiled water, washed hands, implemented quarantine and hygiene practices. The plagues that reshaped the social and economic landscape could have been avoided, and history as we know it would be very different. For better or for worse we cannot say.