Learn English – ‘explain like I’m five’

The word summary in your example is not a noun, but an adjective which means:

Law (Of a judicial process) conducted without the customary legal formalities.

Summary execution means:

an execution in which a person is accused of a crime and immediately killed without benefit of a full and fair trial.

You could find more about summary execution in the Wikipedia article.

[Oxford Online Dictionary]

Here is a key.

And here is a lock to which the key belongs.

The key belongs to the lock. Hence, "to which the key belongs".

Here is a bowl. That is the cabinet in which the bowl belongs.

The bowl belongs in the cabinet.

Here is a vase, and that is the table on which the vase belongs.

The vase belongs on the table.

Here is a log cabin, and that is the forest from which the logs were harvested.

The logs were harvested from the forest.