please read about S&R protocol before it
Take an example -
For S&R protocol -
Tt = 1ms Tp = 19.5 ms Bandwidth = 40 Mbps
then efficiency = 1/(1+2a) where a = Tp/Tt
so efficiency = 1/40 i.e. we could have sent 39 more packets when we were waiting for acknowledge for 2*Tp time, but we sent only 1 packet instead.
so basically we are using only (1/40)th of bandwidth, so utilization = (1/40)*40 = 1 Mbps
so here we are sending only 1 packets in compare to capacity of 40 packets, so utilized bandwidth is 1 Mbps only. If we send 40 packets , then we will able to utilize entire bandwidth
So basically efficiency means "how much of total capacity of channel (bandwidth) you are consuming in terms of percentage" and utilization means "how much of total capacity of channel (bandwidth) you are consuming in terms of bps"