What does ECU Reset actually do?
 

Im asking this because when I changed the battery out of my car, it wouldn't idle properly and would just shut off. So, I drove it home (while stalling at every stop) and let it rest for abit, turned on the engine and it would idle properly. However, when i give it a little rev, it would turn off the engine straight away...
So I heard that I had to reset my ECU for the car to get used to the fuel trims, but what does the ECU reset actually do? would it straight away make my car stop from stalling at every red light/stop sign? Or should I continually just drive the car around while it stalls at every light hoping it would adapt to the fuel trim?

well one thing a reset does is set the fuel trims to zero. so if your car wanted fuel trims that were not zero it takes a while for it to relearn.

additionally there are three fuel trims, the ECU uses an airflow number, but baiscally they have an idle trim, 1500-2000ish trim and 2000+ trim

so for it to learn an idle it needs to be near idle.

if the fuel trims are off, it might be an indication there is something wrong too. the ECU is actually really smart and will compensate for all kinds of things

Try this first:

https://lh5.googleusercontent.com/-H...219_205421.jpg

If you are stalling after that procedure, something else is wrong with your car. Stop driving it until you can sort it out properly.

im sorry what does changing your battery have to do with your fuel trims?
you probably left your maf unplugged