Twu Wuv is True Love, but not really. More accurately, it's a portrayal of 'true love' that is unworkable or otherwise unbelievable. While the question of if there really is a 'true love' can be debated, it is undebatable that many badfic characters and possessed canons try and use it to explain implausible relationships. The phrase 'twu wuv' originates from the speech of the priest in the movie version of The Princess Bride, which is a story all about more well-executed True Love.
Twu wuv can become a charge, because it mostly only appears in badfic, although the Twilight series is famous for portraying twu wuv in a formally published work. Even fairytale true love encounters obstacles.
Characters in twu wuv love each other simply 'because they do,' often after merely seeing each other. Often, their demeanor changes completely soon afterward to make the romance good and pure, hitting no obstacles such as incompatible characterization or issues in backstory. Characters that interfere with twu wuv are often punished, and characters that praise twu wuv (even if would be inappropriate for them to approve of the relationship) are often rewarded for being 'right.' Twu wuv often solves all the problems in the story, even if it encounters superficial resistance.
Never mind that emotional attachment and romantic goals are one of the leading creators, not solvers, of plot and conflict in fiction.
More often than not, twu wuv comes across as hormonal and shallow. Many 'twu wuv' relationships are heavily based on image. One of them sees the other, falls in love with their beautiful eyes or perfect body, and somehow everything works out so that they both are happy with each other. Nonphysical characteristics are sometimes also involved in twu wuv, but it's often limited to surface mannerisms or hurt/comfort concerning angst (the romance always curing the angst, of course).
If you read the above description and identified the emotion captured as lust, not love... you would be right! Twu wuv is an immature mindset, mistaking lust, hormones, and desire for attachment for love. It can be seen time and time again in doomed young romances in the real world, so it is little wonder that so many immature or young Mary Sues fall into the twu wuv trap. This takes a more horrifying turn when you consider the sheer power Mary Sues and Suethors have over characters and worlds; unrealistic romantic expectations cause hundreds of unsupportable pregnancies, heartbreaks, and other problems every year. Applying logic that doesn't even work in the real world to others against their will is selfish and negligant at best, frightening at worst.
As for usage itself, the phrase 'twu wuv' is often used to poorly conceal disdain of fangirls who will readily proclaim their lust object to be their one true love, often in babytalk or chatspeak. It can also be used to display the same disdain for the characters fangirls come up with to act as wish-fulfillment.