Translation:This is serious.
I'm not a native speaker but the dictionaries seem to give more example for grave that relate to grave matters, i.e. things that are worth taking seriously. For sérieux, the examples seem to be for describing something that takes things seriously or has a serious, as opposed to frivolous, approach.
There are different forms of "ce". It may be a demonstrative pronoun meaning "it, this, that". It is always considered to be masculine when followed by an adjective. That is the use in "C'est sérieux". "Ce" (m) and its variations "cette" (f) and "cet" (m, before a vowel or mute H) are demonstrative adjectives meaning "this" or "that", e.g., ce livre = this/that book, cette pomme = this/that apple, cet homme = this/that man. https://www.thoughtco.com/french-demonstrative-adjectives-1368790
I had this as part of a multiple choice question (where you select each word in order as part of your answer). Just to see if it was accepted I worded it "is it serious", as a question, rather than "it is serious". I was marked wrong but I believe should've been marked correctly as French questions can be worded / ordered in this way. Thoughts?