Poland and the Baltic States (Estonia, Latvia and Lithuania) have asked for some American troops. Not enough to halt a Russian invasion, just enough to ensure that the Americans and their NATO allies (or at least some of them) will intervene if Russia does attack. These four nations already have a mutual defense guarantee from NATO in the form of NATO membership. But that is not enough and what has been asked for, and granted, are some American troops in each of these nations. Not many, there will only be one reinforced battalion per country. That means about 4,000 troops overall.
These four East European countries join a growing list of nations who, threatened by dangerous neighbors, have agreed (and often asked) to host American troops. The first and most obvious examples of this are South Korea, Japan and Germany, where the host nations even pay for some of the costs for the foreign troops to be there. This form of defense has been quietly followed by a number of nations in the Middle East, like Saudi Arabia, Kuwait, Bahrain and the UAE (United Arab Emirates). All of these Persian Gulf nations want the Americans around to keep the Iranians out. But it is not just the Iranians. Inside Iraq there have been American troops in northern Iraq to protect the autonomous Kurdish majority up there from the Arab majority.
This form of security is also called a "tripwire force" because if the host nation is attacked the presence of some U.S. troops means that a lot of U.S. reinforcements will promptly arrive. Some nations kick the Americans out once they believe they are safe. Thus the Philippines asked the U.S. to depart in the 1990s after the Cold War ended. But as China became increasingly aggressive a decade later the Philippines has asked the Americans to return, which they have agreed to do. Several other nations are seeking this form of security guarantee but are not getting it, at least not yet. This includes Ukraine and Georgia. The United States is the favored source of these armed hostages because the U.S. is a super power and, compared to all the alternatives, the least likely to take advantage of the situation.