These are called Merge Fields, they are a way for you to insert dynamic field values from the current record inside a static text object on your layout. These are then not editable by the user and are useful for creating letters and things where the you want to mix static and dynamic text.
You can add these in you selecting 'Insert > Merge Field...' and then specifying the field you want to place on the layout.
You can also format Number, Date and Time as you would a field object.
I hope this makes it a bit more clear for you.
Very clear. Thanks.