Can someone explain what “rangefinder” means in this context?
Its used to find the distance to target.
You need this to correct for bullet drop.
A rangefinder is a handheld laser device used by long distance hunters and competition if allowed to get range to a target within a few inches or feet. The higher end ones even do inclination and azimuth which for really long distance the earth’s rotation matters. There is only one purpose for that device it should have been seen as a imminent threat.
https://www.sigsauer.com/electro-optics/rangefinders.html
When you fire a bullet, it will drop over the course in accordance its speed until it hits the ground. A range finder allows you to measure the distance so you know how high to aim your gun so the bullet hits the target instead of the ground.
It’s pure physics that, once you know the distance and elevation you be very accurate in hitting your target.