Some layers, especially reference layers, may have odd names. In our example we have a layer called "ne_10m_admin_0_countries". As the creator of this map, I know that the dataset name means "Natural Earth, 10 Meters, Administrative Boundaries Level Zero, Countries". However, those that want to look at my map will probably want to see it say "Countries".
To rename the layer, select the layer in the right hand side, and click "Layer Settings"
Once the layer settings are opened, click the vertical ellipse next to the name, and select "Rename Layer" and rename the layer. After renaming click the save icon.
That's it. You've now renamed your layers so that your users don't have to decipher idiosyncratic names.