What issue is Patrick Henry talking about that is "nothing less than a question of freedom or slavery?"

Although Patrick Henry delivered his “Give Me Liberty or Give Me Death” speech before the Declaration of Independence had been written and signed, the issue that he addresses in this speech is, essentially, the issue of American independence.  He is saying that the colonists can fight (and risk dying) for freedom or can choose to live in slavery. 
Henry delivered this speech in March of 1775.  The colonies had, of course, not actually declared independence.  The battles at Lexington and Concord had not yet occurred.  The Virginia Convention was not discussing whether to declare independence.  However, this was the issue that Henry was talking about.
The actual subject of debate was whether to organize militias in every county of Virginia to fight, if needed, against the government.  Henry had proposed this, but some delegates thought that it was a step too far or that it was too provocative.  Henry did not agree.  He argued that the colonists had tried everything they could to resolve their issue with the British government.  Now, he argued, they had no choice but to get ready for war.  It is clear from his words that he believed the colonies were fighting for their freedom and independence.  He said

If we wish to be free-- if we mean to preserve inviolate those inestimable privileges for which we have been so long contending--if we mean not basely to abandon the noble struggle in which we have been so long engaged, and which we have pledged ourselves never to abandon until the glorious object of our contest shall be obtained--we must fight!

In this passage, he is arguing that the colonists have been struggling for freedom for a long time.  He says that they must either continue to fight or they must “basely” (meaning in a cowardly or dishonorable way) give up.  When Henry says this, it is clear that the issue he is addressing is the issue of American independence.