Every line of output in the disassembler represents one word of memory, which can be a workspace value, program data, or an instruction of the actual game code. Each word is refererred to by its address, which is shown in blue on the disassembler between the interpretations of the word as data and as code.
Normally, to find cheats you search through the workspace values to find the addresses which hold the values for number of lives, time, or whatever else you wish to find. By altering the contents of these addresses you can gain more lives or time, or anything.
Then, if you understand ARM code you can search through the game code to find the code that affects a value in the workspace, and find an alteration to make to the code to achieve effects like infinite lives, or better firepower.
It is also possible to search through the program data, for example to try to find passwords.
Putting the cheats into action
Using the in-game features