In plain English it is knowing and defending your faith.
A deeper explanation would be a theological science which has for its purpose the explanation and defence of the Christian religion.
Apologetics means, broadly speaking, a form of apology. The term is derived from the Latin adjective, apologeticus, which, in turn has its origin in the Greek adjective apologetikos, the substantive being apologia, "apology", "defence".
The plural form, "Apologetics", is far more common and will doubtless prevail, being in harmony with other words similarly formed, as ethics, statistics, homiletics. In defining apologetics as a form of apology, we understand the latter word in its primary sense, as a verbal defence against a verbal attack, a disproving of a false accusation, or a justification of an action or line of conduct wrongly made the object of censure.