A comfort ship is a ship that when you see content of, it can help boost your mood. It can also be a ship that you hold very close to you, and that you love so much that it radiates comfort. Basically, it's just a ship that gives you good vibes from their chemistry and interactions <3. There's also a good chance someones comfort ship might also be their OTP.
Note; A comfort ship doesn't need to be one without any angst. There are a lot of ships people say are their comfort ships that end up not happening and/or have angst in them. Nor does a comfort ship need any sad themes to be one.
Person 1: "Is this your comfort ship?"
Person 2: "Yup! It is <3. I love those two so much!!"
When a character is mischaracterized in a way that removes any red flags or when their only character trait is being comforting.
"Hero (OMORI) is an adult right? Why does every fanfiction I read write him as a comforting parental figure instead of a young adult learning how to be an adult?"
"He's a comfortshield. Some people don't like giving him red flags, or substance."