Yeah when you're playing an instrument in a symphony you have a very, very narrow ability to hear and understand everything that's going on. Your own instrument is (usually) in your face, you might be in a section with a bunch of the same or similar instruments that is drowning stuff out, everyone is facing away from you and the acoustic echo is weird, etc.

Conductor stands right in the middle of it all and can actually hear everything. A conductor can guide entire sections, or even easily pick out a specific player and get them to be louder, quieter, slow down, etc.

Each player in the symphony is paying attention to that person and they all take cues from them. It's pretty wild.