Journals either embrace them — or hate them.
If a journal’s guidelines say they accept simultaneous submissions, this means you could go ahead and send those same poems, or some of those same poems, to another journal for their consideration as well. It gives you permission. It is not a requirement.
If the editor says no to simultaneous submissions, do not under any circumstances send them elsewhere. This means you have to keep good records. Some editors take nearly a lifetime to reply to your submission. (Others, thankfully, are a little speedier.) Know what you’ve sent where.
You should only send your poems to places you want them to be published. Even if you simultaneously submit, the first journal that comes back to claim your work should make you happy. You should say yes, gleefully, to that journal. For example, if you want to be published in The New Yorker (and who doesn’t?), I believe you should send only to them, and wait until they say yes or no, because it will be an absolute drag to have to revoke your poems when there’s still a chance.