UML Deployment diagram, what does the deploy dependency mean?

When you have a dashed arrow with the <<deploy>> keyword, one end will be a Node and the other end will be a DeployedArtifact. This indicates that the Node can support the DeployedArtifact and is an alternate representation of putting the DeployedArtifact symbol inside of the Node symbol.

Here is a graphical representation of the two options, from page 659 of the UML 2.5.1 specification:

In both cases, the artifacts have been deployed onto the node.

I think that it would be a safe assumption that if an artifact has been deployed to a node, then it is being used on the node. If it's an executable, then the node will, at some point or under certain conditions, execute it. If it's a configuration file, then the node will, at some point, read it and use it.

Typically, I've shown multiple executions of a specific artifact in the textual description around the UML model. There may be other ways of doing it, such as multiplicity on the <<deploy>> relationship or including multiple instances of the artifact within the node, but I've found that these get confusing to readers. Simply showing that an artifact lives on a node and providing other text or tables around the number of instances is more straightforward.