A two-bagger is an investment that has doubled, that has risen 100% or more.
We believe it was Peter Lynch whose seminal book, One Up on Wall Street, popularized the concept of baggers, borrowed from America's national pastime of baseball. In baseball, a double is often referred to as a "two-bagger." The same parlance is used, by extension, to refer to an investment that has doubled.
Newer Fools often ask the question, "When my stock doubles, was that a one-bagger or a two-bagger?" It seems counterintuitive that you'd get a "one-bagger" just for showing up -- just for buying in the first place -- which is why this is a frequently asked question. And also, a double is a 100% gain, which also looks visually wrong, as you'd think a two-bagger shouldn't be 100% but 200%, but of course it is just 100%.
Just remember that a double in baseball and in investing is a two-bagger. You'll have no problem using the phrase; indeed, you'll have quite a lot of fun using the phrase when you achieve a double!
Similarly, a three-bagger is when you triple your money, etc.
A two-bagger is a stock that doubles, gaining 100%.
A three-bagger is a stock that triples, gaining 200%.
A four bagger is also considered a home run 300%