An embargo means that the release is scheduled to be public on a specific day and time in the future — it is “embargoed”. This is sometimes done for a big announcement or a product launch press release, to give a PR manager or marketing manager time for advance outreach.
If the news is embargoed for a specific day and time, that should be prominent on the release or otherwise made clear. If there is an embargo, it is best to list when it lifts in red letters on the face of any advance press release. Emails get buried and the journalist may forget about it if solely in an email.
Another important point: ask the media outlet if it will honor an embargo, before sending the press release out. Most do honor embargoes, but some do not. Only send an embargoed release once the outlet confirms.