Q: What does the status "Reviewer assigned" mean for a Springer journal?

The status of a manuscript can differ across journals and publishers. Since the status “Reviewer assigned” appeared just a few days after submission, I think it refers to the internal review. Possibly, your paper has been assigned to an editorial assistant who will check it to see that it meets the journals requirements. Once this is done, it will be assigned to an Associate Editor who will be in charge of your paper for the rest of the journal evaluation process. He/she will first review your paper for a match with the journal’s scope, the quality of the paper, and the novelty of the study and decide whether to desk reject it or send it out for peer review. The peer reviewer would assess the soundness of the science.

As it has not been too long since the last status change, you should wait for some more time. If there is no change for another month or so, you could write to the journal politely seeking an update.

Related reading:

• What does a status change from "with editor" to "under review" mean for a Springer journal?
• What does a change of status from 'Under Review' to 'Editor Assigned' mean for a Springer journal?