Labels are used to create groups of nodes. A label is just a text string that you attach to a node. On the Nodes page you can add a label to a Node by clicking on the plus sign (+) in the labels column next to the node. You will then get a list of existing labels you can select from, or you can add a new label by typing in some text.
When deploying flows from the Flows page you can deploy to multiple nodes by selecting one of these labels on the Labels tab in the deployment dialog.
Labels can be removed from individual nodes by clicking on the cross mark (X) on the label in the Nodes table.
Labels can also be removed completely on the Labels tab of the Nodes page. If you delete a label here it will be removed from all nodes and from the list of available labels.
By selecting a label on the Labels tab you can also get a list of nodes with that label attached.
When defining roles on the Organization page labels can be used to limit access to certain nodes. If you expand the Nodes group in the role editor, labels can be added to all of the nodes permissions. Then a user with this role will only get the specific permissions for nodes with the selected labels.
Note that you need to limit label management if you intend to use labels to control node access, otherwise someone could just add the labels needed to get access to any nodes.