In a nut shell, route summarization is the process of summarizing subnets into a larger subnet to be advertised to upstream routers for ease of management/troubleshooting and resource conservation.
However when working with OSPF you can only summarize at an ABR or ASBR. There are two types of summarization in OSPF; Inter-Area Summarization and External Summarization(LSA 5).
Inter-Area route summarization can only be done on the Area Boarder Router (ABR) and summarize routes from a particular area into the backbone area by using command area
External Route summarization can only be done on an ASBR and summarizes routes by using "summary-address network netmask" command.
In your case you can run the command on ASBR to perform the summary.