How did Redi’s experiment help disprove the idea of spontaneous generation?

Louis Pasteur was actually the one who disproved spontaneous generation, however Francesco Redi was one of the people who tried to disprove spontaneous generation.

In 1668 Francesco Redi set an experiment where he put decaying meat on two jars: One was unsealed and the other one was sealed

Since flies cannot lay their eggs on the decaying meat placed on the sealed jar, no maggots appeared on the meat in the sealed jar. However many scientist said that air is needed for life to arise from non-life so for the second time he set up an experiment with three jars with decaying meat: first was unsealed, the second one was covered with gauze and the third one is sealed

http://parkviewbio.blogspot.com/2015/08/month-11-science-of-biology.html

The result is that maggots appeared on the first jar that was left unsealed because flies were able to lay their eggs on the meat. It was a big blow on the belief of spontaneous generation but many scientist still believed that "animalcules" described by Leeuwenhoek is small enough to be created by non-living materials and can pass through the gauze.

Fast-forward through time Louis Pasteur set an experiment where a broth was boiled and placed in a flask. The neck of the flask was bent. Microorganisms in the air was trapped in the neck of the flask and the broth was free from microorganisms

Pasteur’s work provided evidence that microorganisms cannot originate from mystical forces present in nonliving materials. Rather, any appearance of “spontaneous” life in nonliving solutions can be attributed to microorganisms that were already present in the air or in the fluids themselves. (Tortora et al 11th ed)

http://creationscience4kids.com/louis-pasteurs-swan-flask-experiments/

Reference:

Tortora, Gerard J et al. (2013) "Microbiology An Introduction 11th ed"