"Aggregate", as already explained by @Leo Laohoo, means the total forwarding capacity of the router, but there are some very important conditions and notes when using a boost license (which, BTW, is just removing the software performance limits - basically the same as prior gen ISRs that didn't have software performance limits).
Firstly, there's mention to reach this performance, only built in Ethernet ports might be used. (Why? Because these routers often have less physical bandwidth to their modules.)
Secondly, often this throughput is based on an IMIX traffic with no additional features (like NAT, ACLs, QoS, crypto, etc.) being used. (Why? Additional features consume processing resources reducing throughput capacity. Also, smaller sized packets, rather than used by IMIX, also consume additional processing resources.)
A critically important couple of words used with the boost license is "up to".
If you want a 4k than can (almost always) guarantee 2 Gbps (nb: enough for just two duplex gig ports), you'll want a 4k that has a 2Gbps, or better, performance license.
However, if you're sure your traffic mix and/or configuration will not be more resource demanding than basic IMIX, a boost license would be fine.
Lastly, I'm replying from my phone, so difficult to provide attachments, otherwise I would attach a nice Cisco white paper on router performance impact of packet sizes and feature usage to throughput (10x delta not unusual between worst and best) and some Miercom performance test results.
