89 : line number for editing or ACL name, depending on platform
permit : permit the following packets
tcp : packets need to contain TCP
host 192.168.10.2 : single source address
any : destination address
eq www : qualifies the any for destination port 80 (HTTP)
So, if this ACL entry (ACE) is applied ingress from the source address or egress towards the HTTP ...
That depends on how the ACL is applied to interfaces. In your case, you're probably right; but you should examine the rest of your config to be sure.
Here is a good primer on Cisco ACL configuration and the relevant reference material on the related ip access-group command, which is used to apply an ACL to an interface.
The reason it cannot be 100% utilised is that there is access contention. The stations are not all coordinating with each other (through a side channel) to use the slots one after another.
You say "if only one station is transmitting in one time slot then there will be no collisions .." No, there will be collisions. Stations will try, and there will be many ...
Generally, only one station can - successfully - transmit in one time slot. Concurrent transmission attempts cause collisions which in turn cause delays.
Statistically, when there is access contention, the probability is 1/e ≈ 36.8% that a frame is successfully transmitted in any given time slot, so that's also the efficiency of the channel. With no ...