Kaycee,
"Spontaneous generation" was the belief that living things could generate, or be created, from non-living things. For example, maggots could be created from rotten meat, or flies could be created from manure.
We know now that this doesn't happen - that living things can't just suddenly be created from non-living material. But way back then they didn't have accept that. All they knew was that the rotten meat was there one day, and the maggots "just appeared" the next day (or next hour....).
When scientists started developing improved equipment and doing experiments following the scientific method, they realized that living things only came from other living things. For example, the developed "germ theory" and started to understand that there were microscopic organisms that were responsible for some of the things they saw.