Sometimes displayed as Ins, the Insert key is a located on most computer keyboards by the backspace key. It is used to toggle between insert mode and overtype mode, which change what happens when you type text in a program.
For example, if the I-cursor is placed in front of any text, as you type, text is inserted and nothing is overwritten. However, if you press the Insert key and the I-cursor changes to a block cursor, as you type the text is overwritten. Not all programs allow you to change the insert method with the insert key. If the cursor does not change when you press Insert, text input mode toggling it is not supported.
In the example image, you can see the different cursors. In the top portion, if we were to type "test " in front of "Computer Hope example" it would become "test Computer Hope example." However, if we pressed the Insert key to change the insert method, the result would be different. Typing "test " in this instance (bottom portion of the image) would make the text "test ter Hope example" because the first part of the text would be overwritten.