It means that 8 is the last point that something will arrive. Using 'by' means up to that point. For example 'I will finish the project by Friday' means that it will be finished before Friday.

"Arrive by 7:00" means "arrive no later than 7:00." You can arrive at 6:15, 6:45, 6:58, or 7 on the dot but, please, arrive no later than 7:00. Do not arrive at 7:20 or 7:12 or even 7:01. Although literally "arrive by 7:00" could mean any time before seven, in real life the normal context would be to arrive shortly before 7:00. Something is going to happen at 7:00. If you arrive later than 7:00 you will miss it, so arrive by 7:00. If you arrive earlier than 7:00 you will have to wait. If you arrive a lot earlier than 7:00 nobody will be expecting you yet, so don't do that, either. To say "arrive at about 7:00" you would say "arrive at about 7:00," "arrive around 7:00," "arrive seven-ish." Although it means "arrive no later than 7:00" it is not a very strong or strict way to say it. If it is really important to arrive by 7:00 and not a second later, you would add other words to strengthen it--usually giving a reason. "Arrive by 7:00 sharp, we will close the doors at 7:00." "Be sure to arrive by seven, we will not hold the bus for latecomers."

than

"arrive by 8:00" > no later then 8.00pm