Aggregate Rate and Poisson Process in Aloha
The dictionary definition of aggregate might be helpful: https://www.wordnik.com/words/aggregate

You have $N$ senders.  Each sender sends packets on a network at some rate, say, $\lambda/N$.  Now think about all the packets that are sent over that network, without regard to who sent it.  What's the rate at which such packets appear on the network?  That's a total over all packets.  The aggregate rate will be something like $\lambda$.

It might also be helpful to refresh your understanding of what a rate is.  A rate is an amount divided by a time period: e.g., the number of packets sent by Alice per unit time interval.  The aggregate rate is also a count divided by a time period, but in this case, the count is of a different quantity: it's a count of all packets, not a count of the packets sent by Alice.  So, the number of packets sent (in total, from anyone) per unit time interval is a rate.  That is what your book is calling the aggregate rate.