Journalists explained what this means.

Several United States military bases in Europe have been placed on high alert due to a possible terrorist threat.
Two U.S. officials told CNN that the security level of bases, including the U.S. Army Garrison in Stuttgart, where European Command is headquartered, was raised to the highest level over the weekend (Charlie). It is used in the event of an incident or intelligence receiving information about the likelihood of terrorist action or attack.
The interlocutor, who is located at one of the European bases, said that they have not seen this level of threat for “at least 10 years.” According to him, this usually indicates the receipt of an “actively credible threat.”
U.S. European Command spokesman Cpl. Dan Day declined to comment on specific levels of protection, other than to say that they continually evaluate factors that impact security and often take additional measures to ensure the safety of service members.
(C)UNIAN 2024

Biden, wake up!
Biden, old bean, just resign. Let a younger one ‘fight’with Trump. Not Kamala Harris off course.
We are still one gear behind in the incremental response to Moscow threats. It is reality. This has an effect of amplifying the threats from the Nazis in the Kremlin who know that they can constantly increase the pressure.
As long as we do not reverse the process, we will be in a position of weakness and opening the doors to large-scale terrorist acts, more cruelty and violence in Ukraine.
Biden still seems like a smart person. But does he realize that the current soft strategy towards Moscow is leading Ukraine to disaster and potentially endangering the entire global balance? I don’t understand. Challenges await us and it is not up to “us” to choose to refuse to take them on. We cannot refuse the state of the world, the reality of wars and their dynamics. Unfortunately, we will not solve all this with memoranda and false peace plans, because the violence of the Nazis against us is massive, organized, motivated. You will not solve anything with the best intentions in the world, but with the right military and strategic tools. It is the war of our time that we must accept like those experienced by our elders.
The risk of escalation is absolutely irrelevant once we assess the threat and the options we have left to defend Ukraine and ourselves.