A role-playing game, or RPG, is a shared storytelling experience. You and your friends get together and create an open-ended story, more like a television series than a movie. One person becomes the narrator, setting the stage and creating challenges for the players. Everyone else creates characters, heroes who go on adventures.
An RPG system is the set of rules the game uses to build characters, set challenges, and otherwise govern the world. Players can choose what their characters are going to be like, their strengths and weaknesses, and the narrator uses the same rules to determine the difficulty of various tasks that come up during the story. One player might decide to make a very strong character, which means they are more likely to climb a sheer cliff the narrator has determined is very difficult to get up.
Tango is a very simple role-playing system that can be adapted to any genre. It has a modular design that allows for just about any degree of customization. It's intended to be light and quick, allowing narrators to easily generate adventures and letting players whip up characters in just a few minutes so they can get right to the action.
Tango uses all the classic gaming dice: four (d4), six (d6), eight (d8), ten (d10), twelve (d12), and twenty-sided (d20). If you don't have the proper dice, you can use a random number generating system or a smartphone dice app. For character sheets, you really just need a scrap of paper.