explaining serial.begin(9600)?

i have ir remote , ir receiver.
i have managed working without problems on arduino mega.

i have programmed it's speed to:

serial.begin(9600)


i have figured out communication speed arduino , device in bits.

the question is:
what decides transfer speed? if choose set @ 57600 instead, how can make sure devices can keep up? trial , error thing?

if set speed "too low" make safer connection, in fewer dropouts?

if speed high, happens? lose communications or hacked message, sender can't send fast enough?

the sender , receiver have use same baud rate or receiver garbage.

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