Squick is a term used to describe the uncomfortable feeling that some people may have when they hear about or see certain kinky acts. This can apply to acts such as bondage, whipping, caning, scat play and blood play just to name a few.
The term squick may have originated in the BDSM community, possibly on a Usenet group focused on bondage. The term is likely a combination of the words "squirm" or "squeamish" and "ick." The term was popularized over time and is widely used in the BDSM community to describe acts that may elicit a gut reaction of repulsion without making any moral judgment about the activity. It has also been adopted by some other communities, including fan fiction.
In general, BDSM includes a very wide range of potential activities, some of which are likely to be unappealing to any one person. So, in response to a question or request about a certain act, a person can say, "That squicks me" or "That squicks me out." It's a way of expressing a hard limit without judgment for other people's enjoyment of it.