Knowledge is something you acquire through study and living.
You find out to use a spoon, how to speak and how to laugh. You need all these things for living and for writing. Some knowledge is specific to a certain job and some knowledge is specific to people. You need all you can acquire or remember when you write.
The mantra of every writer seems to be “write what you know”. That could put you in a situation where you only wrote about babies, maybe even just boy babies, because that is what you know.
I the write what you know can mean not facts, but feelings. You know what it feels like to be awakened at 2 am after having been up at 12 am til 1 am feeding the baby, and baby wants food again. Take that feeling and put to a character who has been up all night finishing their college paper and the baby wakes at 2 am for feeding. It is annoyance, frustration, and love. You know you want sleep, need it, and can’t have it and you love the baby.
Writing what you know can mean that you have to research setting, jobs or education, in order to know what you want to write about.
Maybe your hero is extremely rich and you have now idea how that feels. If you can visit places the rich frequent or perhaps the house of a rich person, you will get the feel of the luxury and everydayness of that luxury. For them luxury is an everyday affair.
Or perhaps, one of your main characters is homeless. Visiting shelters, dressing like a homeless person would, hauling everything you need daily in a cart or backpack could give you that feel.
If you have ever felt pampered you can use that emotion in your rich person. If you have ever felt true hunger and known that you could buy nothing to assuage that hunger, tap into that for the homeless character.
Knowledge of grammar and spelling is necessary to your writing so that others may understand it.
Knowledge is key.