offline
Not connected to the internet or another computer network.
Ich muss den Artikel jetzt speichern, da ich im Zug offline sein werde.
I need to save the article now because I'll be offline on the train.

Loading your saved state.
Not connected to the internet or another computer network.
Ich muss den Artikel jetzt speichern, da ich im Zug offline sein werde.
I need to save the article now because I'll be offline on the train.

As an adjective, 'offline' is almost always used predicatively (after a verb like 'sein' (to be)) or as an adverb. It is normally not declined before a noun.
Mein Computer ist gerade offline.
My computer is offline right now.
The word 'offline' was borrowed directly from English. It literally means that you are separated 'von der Leitung' (off the line), meaning you have no connection to the network.
Ich speichere die Datei lokal, damit ich sie auch offline bearbeiten kann.
I'm saving the file locally so that I can also edit it offline.
In the meaning 'reale Welt' (real world), 'offline' is often combined with verbs of meeting and communication. Typical phrases are 'sich offline treffen' (to meet offline), 'etwas offline besprechen' (to discuss something offline), or 'jemanden offline kennenlernen' (to get to know someone offline).
Nach monatelangem Chatten haben sie sich endlich offline getroffen.
After months of chatting, they finally met offline.
online
'online' and 'offline' are opposites. 'online' means that someone or something is connected to the internet, or that something takes place on the internet. 'offline' means that there is no connection to the internet, or that something does not happen in digital space. The words can be confused because both are often used for courses, meetings, games, or devices. The simple guideline is: with the internet or on the web means 'online'; without the internet or in person means 'offline'.

Am Montag ist der Kurs offline, weil sich alle im Büro treffen.
On Monday, the course is offline because everyone is meeting at the office.
Für den nächsten Tag habe ich den Bericht online abgegeben.
For the next day, I submitted the report online.