“Multiple submission” means sending your manuscript to more than one journal at a time to try to get published by one of them. Never submit your work to more than one journal at a time.
This might seem like a good shortcut. Maybe you think you can save time and increase your chances of acceptance. But never do multiple submissions. It is unethical. It can create copyright problems. Plus, you are wasting the journals’ time and resources. You must only submit your work to only one journal at a time.
Also, repeated evidence in the literature can bias the overall evidence. There are meta-analyses that try to pool all available data to calculate an overall effect. If you submit to more than one journal, your study may be counted more than once in these meta-analyses. This will skew the data.
Your work may be rejected by all of the journals if it is found that you have made a multiple submission.
If your manuscript is rejected by a journal, then it is acceptable to then submit it to another journal.
You may wish to withdraw your submitted manuscript from one journal and submit it to a different journal. If so, you must inform the editor of the first journal. You need written confirmation that your submission has been withdrawn before you submit your work to a different journal. Do not just assume if you do not hear anything that your original submission has been withdrawn. You need to get a formal notice from the first journal before you can submit your manuscript to another journal.
Always be sure that all the co-authors of a manuscript agree to submitting it to your target journal. This will avoid any possible conflicts. If you decide to withdraw your submission, you also need to have approval from all the co-authors.