Most event are reliable, This means machine A sends a message to machine B, then B sends a “I got it” to A. If the “I got it” (ACK for Acknowledge) does not arrive in a bit of time, then A tries to send to B again.
Unreliable means A send to B and if B does not get it (it gets dropped) then it is no big deal.
Things like voice chat (VOIP for Voice Over Internet Protocol) is sent unreliable because if a 1/10th of a second of voice gets missed it is too late to resend it anyway and you get a gap in the voice. Also player hand position updates are similar. If you miss one update the next one will get the position anyway.
As you can imagine, Unreliable is very efficient and uses less network.