Q: What does it mean if the status of my manuscript changes from "Under review" to "Awaiting decision"?

If I understand correctly, "Under review" was the first status that showed up directly after submission. If that is the case, it probably refers to the internal editorial review (also known as the initial editorial screening), not the external peer review. This, in turn, means that the status "Awaiting decision" refers to the decision that the editor will take about whether your paper will be sent for peer review or desk rejected.

It is difficult to predict the outcome of the manuscript at this stage, but you can be hopeful. If the paper clears the initial editorial screening, it will be sent for peer review. 

Related reading:

Peer review process and editorial decision making at journals