What does "multiplex" mean?

Question

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I keep running into uses of "multiplex" in genetics and genomics papers, and while I get the basic idea of the word in general (I think), I don't really understand what it means in genomics/genetics.

For example, from this paper:

"In addition, many existing digital molecule counting methods such as digital PCR, digital microarray profiling and direct single-molecule sequencing cannot be effectively multiplexed and are thus generally only applicable to measuring one or few molecular species from many samples, or many species from a single sample."

[MSRS]

An example can be given to you with Multiplex Sequencing like

In multiplex sequencing, DNA fragments from different samples are pooled and sequenced altogether. The main reason is to increase sample throughput. The result is a mixture of sequencing reads from different samples. In a follow-up demultiplexing step, the reads need to be separated by using the attached barcode (sample marker) sequences. This demultiplexing step usually is already done by your sequencing core facility.

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So is an assumption here that before mixing, each original species' sample will have an identifying barcode attache?

[MSRS]

Yes, barcoding samples then mixing and after sequencing, extracting each individual sample.

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Thank you!