In the context of the skyfeed builder the regex block is looking for any single match within your whole post i would hope the case sensitive flag is self explanatory and the invert flag makes it so that instead of the filter only including posts which match it's removing them
The "Target" flags are choosing what you want to be searching within, between post text, image alt text, and links. They're non-exclusive so you can choose whatever set of those 3 you'd like.
There are a few special characters to take note of in the entry to the block, "^" is the beginning of the text and "$" is the end for example "cock" will match all of the posts containing cock, "^cock" matches all posts with cock as the first 4 characters and "^cock$" only matches if the whole post is just the word cock
This mini section is kind of a whole tangent specific to individual characters