This idiom refers to somebody or something working or happening together, at the same time, or that are linked in some way.

As the video shows, this expression can be used in a social context, but it is more common in a professional one where two or more people or pieces of equipment, like computers, that are linked or working closely together are frequently said to be 'in tandem.'

Phrases with a similar meaning include 'side by side,' 'shoulder to shoulder,' and 'at one and the same time.'