Looking for info on trigger shot

Hi there

I'm on my second cycle of letrozole 2.5mg and I've just been told I have 2 follicles of the same size in my left and 2 follicles of different sizes in the right. The biggest is in the right side. They've asked me to come back in 2 days for another scan and if they are happy with the growth they would be giving me a trigger shot incase I don't ovulate on my own.

As they were in a rush I didn't get to ask much. My question is which follicle would ovulate first as they've told me there are 4 total. And what does the trigger shot do. My consultant didn't mention the trigger shot so it's the first time I'm hearing about this.

Just looking to educate myself on how all of this works.

Thank you 😊