It is used when you are describing two things that are happening at the same time. You can't say two things at the same time, you have to say one before the other. So, first you describe one series of events. Then you say "meanwhile," and describe the second, paralell series of events. For example: 11 am--I board a train in Norwood. Noon--my train arrives in Boston. 11 am--my son boards a bus in Malden. Noon--his bus arrives in Boston. I could describe it this way: "I got on the 11 am train and went to Boston. Meanwhile, my son was riding the bus into Boston." "Meanwhile" is the literary equivalent of a flashback. It takes you back in time. It introduces a description of something else that was happening at the same time as the things you just described.

It’s stating two different events and can be on two different timelines. I’m thinking about what to cook for dinner tonight, meawhile did you see the latest headlines It can be at the start of a sentence. Just depends of the context Meanwhile can be easily confused with “meantime”

In most cases it means that two things are happening at the same time. Example: We can go get some coffee in the meanwhile. (We can go get some coffee while something else is happening.)

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We will arrange for the item to be shipped. Meanwhile, please arrange payment to the enclosed bank account. Equivalent to "while this is happening ". Sometimes it is used to mean more "also" without a sense of time.