RouteAggregation refers to the collection of "adjacent" routes together, in order to shorten the routing tables. It is made possible with ClasslessInterDomainRouting (CIDR).

If you have enough adjacent routes to use a higher prefix, you can use RouteAggregation. For example, if your network has

192.168.0.0/24 192.168.1.0/24

On different subnets internally, you can advertise the whole lot by using 192.168.0.0/23. A /23 prefix has one less bits than a /24, which is 2 networks - so the 2 adjacent /24 networks starting from 192.168.0.0/24 can be thought of as a single /23

Essentially any CIDR prefix can be thought of as the aggregation of all longer prefixes under it:

192.168.0.0/16 is the aggregation of the 256 192.168.x.0/24 addresses

A /24 prefix is the aggregation of the 4 /25 prefixes under it

See http://jodies.de/ipcalc for a CIDR subnet calculator