To understand it, start with Kaliningrad. It is a bit of Russia that is cut off from the rest. I have coloured it yellow on this map and as you can see, it has a coastline so it is accessible to the Russian fleet, but otherwise it has no direct land connection with the rest of Russia.
Belarus under its current regime has close ties to Russia. So the recent decision by Belarus ask to store nuclear weapons supplied by Russia has increased tensions in the Baltic States. They fear that if Russia decided to command the land connection between Kaliningrad and Belarus then the Baltic States would be isolated from the rest of Europe, and easy pickings.
The land connection is known as the Sowalki Gap (I’ve marked it red on the map_ and Russia has been trying to establish a solid route along it for years – since the break up of the Soviet Union. For more detail of the history of the Suwalki Gap, read the Wiki entry.