What is an SMS trigger?

For every e-mail trigger you have the possibility to add an SMS reminder. If the original e-mail trigger wasn't opened or it didn't generate a sale, you can send an SMS to the client reminding them of the offer available.

Is the text in the SMS standard or I can edit it?

You can edit and write anything you want from the Retargeting admin. You can add a shortlink to a landing page or you can send a voucher; you can send anything you want.

Can I send an SMS to all of my subscribers?

No, you can send this only as a reminder to one of the triggers. This is an SMS trigger, not an SMS campaign.