Folding hands which are statistically likely to lose you money is one of the core competencies in Texas Holdem which you must master to make long term profit - but is one of the areas which many poker players struggle to handle and where they can make mistakes.
It's often tempting to play marginal hands with the hope of improving a hand where folding is the right strategy to follow, and there are situations you'll encounter where folding even good hands is the right thing to do. Playing a hand you should have folded is likely to be the main reason you could potentially lose money, and certainly folding hands where you have nothing and can't draw to anything is going to be the right strategy in virtually all cases in normal play.
Learning when to fold sits right up with the skills of bluffing and analysing your opponents in importance. Remember that folding regularly and misleading your opponents may also set you up for a strong bluff at a later stage of the game or tournament.
Folding is especially important in the early stages of poker tournaments where survival is the most important aspect in the early rounds, you want to be playing premium hands only and folding poor hands is a must.
Establishing a table image as a player who will fold regularly with weak hands can ensure that your later attempts at bluffing are likely to succeed. You can see from this how folding merges nicely with the skills of bluffing.
Folding before a hand reaches it's climax can also give you another benefit, namely the time and opportunity to study the playing styles of your opponents. This is the time to watch carefully for clues which will help you win future hands.
Additional Resources- Folding
Pokertips.org have a good article which covers the subject of folding in depth.
The video below shows 10 of the most amazing folds in televised poker games you're likely to see.