They interpret whatever's on the page. like sometimes they see subtext or something between the lines that the author may or may not have intended, or they take an event or confrontation and interpret it as the character/subject doing a certain thing. like they can see subtext suggesting that maybe two characters have a budding romance, or that a character crashed the car because they were intending to kill the passengers (excuse the weird example.) it's usually just when it isn't explicitly spelled out. sometimes it <em>is </em>explicitly spelled out and the critic can't see it for what it is. just. yeah. you get the picture
They give their opinions on the deeper meanings of a story.
Literary critics read the entire text and start analyzing it based on several things: the figures of speech, the historical context of the text, the author's background, the style, the era, etc. and then start basing their opinions. All of these things together are combined into a critic's opinion about a particular text, and they have to take all of these into consideration whilst trying to find the deeper meaning.
The book that I am reading is Man in Search for Meaning by Viktor Frankl. the setting is at the Auswitch's concentration camp in the time of World War 2 and the main character is the author himself. The purpose of the book is to find meaning in every situation a human is being exposed to.