All text is contained within a text box or panel, of which there many varieties - whether that's within a template, or as one of the stand alone Text items you can find in the text panel by clicking Text in the left hand toolbar.
Editing text is easy - just click in your text to place a text cursor, and a panel of text controls will appear to the right of the editing area. You can click and drag to select text in the usual way.
The text controls allow the selection of Text Styles and the usual range of text editing controls. The yellow Hi applies a yellow highlight color (text background).
The Text Editor controls the following functions:
The settings in the alignment panel control how a block of text behaves within a text box.
The settings in the top row control the various justify options - left, center, right and fully justified. The settings in the middle row control the alignment options - the text can be fixed or aligned to the top, middle or bottom of the box.
On the left of the bottom row is Fit to container. Auto-fitted text changes its font size dynamically to always fit the text into the text area...
If you change the dimensions of a text item the font automatically resizes to fit the new size and if you add more text to an item the font resizes to fit the available space - like this...
The final option is Shrink text on overflow. This controls what happens to the text when it overflows a text area. So with this option selected text will scale down so that it always fits the text area - useful for instances where you need to add more text yet are confined to a specific size.
If you subsequently drag the text area out to increase the area or if you delete some text then the text size will increase to fill the area but it won't get any larger than the original size.
Check out this tutorial for more info...