The Del key deletes the character under the cursor. All characters to the right of this place are moved 1 place to the left (sometimes the contents of the next lines also, if they belong to a multi-line displayed record which is not our concern here).
The Backspace key, often marked with an arrow to the left, deletes the character before the cursor. As with the other key, all characters to right of this place are moved 1 place to the left.
Inserting characters is a little bit more complicated, because it has to be separated from overwriting of an existing character. It is true that nowadays there are many programs which offer no possibility for overwriting a character but only deleting and inserting, but not so exaEdit.
The restrictions for inserting are the same as for deleting a character.
In order to insert a character in the editor window exaEdit has to be put into the insert mode (please distinguish that from the input mode). This is accomplished in general by pressing the insert key. When the insert mode holds it is shown in the status line by the character ^. As long as the insert mode is valid all characters put in will not serve overwriting at the place of the cursor but inserting at this place. All characters to the right are moved 1 place to the right.
In the default behaviour of the editor the insert mode is ended by pressing the return key (or any function key). If you want to end it before, perhaps because the next characters you want to type are meant to overwrite, you must press the insert key again. Prerequisite is that at least 1 character key has been pressed after switching on the insert mode. We call the insert mode described here the ordinary insert mode (in contrast to the insert mode described in the next paragraph).
Now comes the permanent insert mode: If you do not want the insert mode by every return key you may do the following: press the insert key twice (without any other key in-between). If you press during that permanent insert mode the insert key again it will be switched off. In addition to pressing the insert key there is also the command
INSMODE