Question
what does it mean when sap bubbles out of a plum tree?

Answer
There could be several reasons.  One is a bacterial canker for which there is no cure.  The other is that woodpeckers may be pecking on the trunk.  Look for holes aligned in a row around the trunk.  If you see these, then wood peckers are the problem.  Usually they cause no significant damage.  The tree can take a lot of this type of abuse.  The third reason is some other wound to the trunk.  Here again, there is really nothing that can be done. I hope all works out OK.  Good Luck.