Literally, to "hold" something means to contain it. To "store" something to keep it safely contained for a while. If something is "held in store," you assume that it some point in the future, it will be let out. Figuratively, it means "surprising things that will happen, sometime in the future, to somebody." For example, I recently went to the annual meeting of a group I belong to. Little did I know what the future held in store for me. Toward the end of the meeting, they announced the "Member of the Year" award, and it was me. Receiving the award was a nice surprise that was "held in store" until a certain moment arrived. Less pleasantly, you could say that the people who boarded the "Titanic" in 1912 did not know what the future held in store for them.