Breakdown of Это значит, что он дома.
это
this / it is
он
he / it (for a masculine noun) / it (for a neuter noun)
дома
at home
что
that (conjunction)
Questions & Answers about Это значит, что он дома.
In English we just say "That means he is at home." Why do we need что here?
In English, we often drop the word "that" in sentences like "That means (that) he is at home" or "He said (that) it is cold." In Russian, when you connect these kinds of statements, you can never drop что. It is strictly required.
Is the comma before что always necessary?
Yes! Russian punctuation rules are very strict about this. You must always put a comma before что when it connects two parts of a sentence, such as introducing a reported statement.
Why does the sentence use дома instead of the regular word for house, дом?
Дом simply means the physical building, "a house". When you want to express the location "at home", you must use the special adverb form дома.
Where is the verb "is" in the second part of the sentence?
Russian almost always drops the verb "to be" (am, is, are) in the present tense. So instead of saying "he is at home", you just say он дома (literally: he at home).