“Input Method” refers to the software which converts what you type into what is stored in a document or file. For Myanmar Unicode, the Input Method is quite complicated because the sequence of Unicode Code points stored is not the same as the typing order.
For example, if the user types ေ (U+1031) and then က (U+1000) to give ကေ then it is stored as U+1000 U+1031, although it is displayed and typed in the opposite order.
It is the job of the Input Method to hide all this complication from the user and automate this substitution and rearrangement process.
The only Input Methods which are currently able to allow the user to type without having to know the non-intuitive Unicode order are Keyman (Windows) and the related KMFL (Linux).
M17N also includes an Input Method, but the current M17N Myanmar Input Method requires the user to fully understand the Unicode specification.