Cheat Engine is an open source tool designed to help you with modifying games so you can make them harder or easier depending on your preference(e.g: Find that 100hp is too easy, try playing a game with a max of 1 HP), but also contains other usefull tools to help debugging games and even normal applications.
It comes with a Memory Scanner to quickly scan for variables used within a game and allow you to change them, but it also comes with a debugger, disassembler, assembler, speedhack, trainer maker, direct 3D manipulation tools, system inspection tools and more.....For new users it is recommended to go through the tutorial(The one that come with Cheat Engine, you can find it in your programs list after installing) and at least reach step 5 for basic understanding of the usage of Cheat Engine
It searches for values input by the user with a wide variety of options such as "Unknown Initial Value" and "Decreased Value" scans. Cheat Engine can also create standalone trainers which function on their own without Cheat Engine. Cheat Engine can also view the disassembled memory of a process and make alterations to give the user advantages such as infinite health, time or ammunition. It also has some Direct3D manipulation tools, allowing you to see through walls, zoom in/out and with some advanced configuration allows Cheat Engine to move the mouse for you to get a certain texture into the center of the screen. This is commonly used to create Aimbots.