What did Lincoln mean “opposed the extension of
slavery in the territories
but not the system of
slavery in the South”?

He did not want to oppose the system of slavery, but rather fought upon the expansion of it into further territories.

Explanation:

I believe he was trying to say that he does hate slavery but if he ever wanted to keep both sides united, he said they were for now allowed slaves but buying slaves etc were not allowed anymore. He wanted to battle the expansion of slavery but keep the country united too so I can see why this must have been a great idea.

In a nutshell:

He believed that if he stopped the expansion of slavery, this would ultimately solve the problem of slavery as well as keep the country united as a whole (remember he is not banning the use of slavery but just stopping the expansion of it keeping both sides happy). He believed that with this, slavery would dwindle down over time until it would no longer exist and be frowned upon.

what following?

explanation:

i also think he is correct. i believe they came to washington to protest their rights.