Cheat Engine is an open source tool designed to help you with modifying single player games running under window 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, and helps you protect your system by letting you inspect memory modifications by backdoors and even contains some ways to unhide them from conventional means.
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.
Besides these tools it also comes with extensive scripting support which will allow experienced developers to create their own applications with easy and share them with other people
For new users it is recommended to go through the tutorial(The one that comes 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
Note: Some anti-virus programs mistakenly pick up parts of Cheat Engine as a trojan/virus.