Spoilers for "the Last Wish" (basically a full recap) below:
The story "The Last Wish" is about Geralt and Dandelion accidentally releasing a djinn. Danelion thinks it's going to grant his wishes, but Geralt thinks it's going to attack them. He tries to banish it using an incantation he heard but doesn't understand. The djinn roars and flies away. Geralt goes to the nearby city where Yennifer is staying, to seek her advice.
In that first meeting, he starts falling for her, while she is simply curious about him as a witcher. Hearing his story about the djinn, she plots to capture it, as it is immensely powerful and could be used to fuel her magic. She knows it will come back for Geralt and Dandelion, so she has them imprisoned to lure it back.
Geralt is talking to the guard and expresses an offhand "wish you would burst", and the guard actually does burst. This allows Geralt to escape, and makes him realize the djinn actually was granting his wishes. He has one left, and goes to stop Yennifer from trying to capture the djinn.
He finds her struggling with it, and we don't know if she has enough power to win or if it will break free and kill her. She tells him to make his third and last wish, which will free the djinn from him and allow her to trap it. He fears that she is too weakened by the struggle and will be killed if he frees the djinn. He comes up with an idea for that last wish that will also save her life, and makes it.
It works. The story does not allow us to hear the actual wish itself, but another character speculates about "tie his fate to hers", and Yennifer hears it and comments about "you've condemned yourself to me". Presumably it was something like love or binding them together. This begins their... let's call it "tumultuous" relationship which continues in the following stories.
In a humorous touch we learn that the "incantation" Geralt attempted initially translates roughly to "get out of here and go f-ck yourself". This was his first wish to the djinn and explains why it was so angry.
Answering the details would be a minor spoiler, this symbol is explained in an early main quest. You can't clear this before you complete that main quest and receive a particular item.
During the quest "Wandering in the Dark" you encounter illusions like this and Keira gives you the eye of nehaleni, which allows you to dispel any illusion
Best Answer
I did a few experiments, and the answer depends on which kind of food it is. I found three different categories, though there might be more:
Each category has a different rate of health restoration, food has the highest at around 100/s, non-alcoholic drinks have a much lower value at around 30/s and alcoholic drinks have the lowest at around 20/s. Alcoholic drinks also have shorter restoration periods than the value listed in the inventory, that value is for how long the intoxication and screen blurring lasts, the 25s beers seem to have 15s of actual HP restoration time.
Ond food and one drink item stack, but not more. Non-alcoholic and alcoholic drinks don't stack, you can have only one active at any time.