You will see a table with three columns. The second column edits the global rules, the third one is for local rules. Each of the rows in them shows three bars once you hover over one. Green = allow, grey = noop, red = deny. (Noop is the setting to not overrule the adblocker lists.)
In the "all" row at the top, click on green in the global column, to allow everything everywhere. Click the lock icon. Now uBlock won't interfere at all.
On a site where you want to block according to the lists, open the dropdown, and in the row with the site's domain name, click the grey bar in the local column. Click the lock icon to save the setting across sessions. uBlock will apply the lists' rules to that site.
If this is unclear, I can make a guide with images.