Cheat Engine is an open source tool designed to help you with modifying single player games so you can make them harder or easier depending on your preference.
If you find that 100 HP is too easy, try playing a game with a max of 1 HP. Or vice versa :)
With Cheat Engine you can make an unlimited health and money cheat, modify character stats or any other values.
Cheat Engine also contains other useful 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:
Besides these tools, it also comes with extensive scripting support which will allow experienced developers to create their own applications with ease 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.
It will most likely not work on online games. Do not use Cheat Engine to violate game rules or licence agreements. You may only use Cheat Cngine for legal activities.