Summarize and give context for the following primary source. https://www.marxists.org/history/cuba/archive/castro/1960/09/29.htm

This is a speech by Fidel Castro that took place in Havana in late September of 1960, not long after the Cuban Revolutionaries had nationalized all foreign-owned property. This particularly affected the US. The speech is noteworthy because a few bombs go off while Castro is speaking. These were attempts on Castro's life by counter-revolutionaries, in all likelihood aided and supported by the CIA.
Castro remains unfazed throughout the speech, responding glibly that:

For every little bomb of the imperialists, we build 500 houses. For every little bomb they make in a year, we construct three cooperative houses. For every little bomb, we nationalize a Yankee estate. For every little bomb of the imperialists, we refine hundreds of thousands of barrels of oil. For every little bomb we will build a plant to give employment in our country. For every little bomb the imperialists pay for, we convert a garrison into a school. For every little bomb the imperialists pay for, we arm at least 1,000 militiamen.

This captures, in the manner of a caricature, much of what the Cuban Revolution sought to accomplish as an anti-capitalist, colonialist and imperialist venture. In many ways, it was quite successful.
Castro continues his speech by discussing the warm reception he got at the United Nations. Only a couple of weeks before Havana, he addressed a speech to and was well-received at the UN in New York. He refers to this in the Havana speech and to challenging the "imperialist enemy":

The imperialist enemy is capable of the unimaginable. The enemy uses any weapons — from the murder of leaders to military invasions, always seeking the murdering hand, the gangster, the pretext. And we should not only be valiant but also intelligent. We must win the battle, we must be victorious against the imperialist enemy. We must win all battles, as we have won in the United Nations.
The imperialist enemy is being defeated at the United Nations. The supporters of armaments, the enemies of peace, the militarists are receiving rough blows at the United Nations. The imperialist enemy must be demoralized before a war. The enemies of peace, those who play with the fate of all humanity, must be defeated on all fronts.

This "imperialist enemy" is quite clearly the US, and we can see the approach of "the murdering hand" and "the gangster" in the bombs that go off during Castro's speech. Still, he remained stalwart and continued that way for over fifty more years.