The phrase "here today, gone tomorrow" is usually used to describe something that is fleeting rather than permanent or long-lasting. For example, a person who feels that he never has enough money may say that his paycheck is here today, gone tomorrow. This basically means he feels the money from his paycheck is gone very quickly and doesn't last nearly as long as he thinks it should. This phrase is an idiom, which means it is used figuratively rather than literally.
When a person wants to describe something as gone or over very quickly, he could use such words as temporary or fleeting. These words, however, do not represent the only options a person has for getting this point across. An individual may also use the phrase "here today, gone tomorrow" to make his point. This phrase means the same thing as the words fleeting and temporary, but allows the person to make his point in a more colorful or expressive manner.
"Here today, gone tomorrow" is an idiom, which is an expression that is commonly used in a non-literal fashion. Usually, such figurative expressions are well known to people who live in certain regions but may be completely unfamiliar in other regions. As such, a person may use it and have no need to explain its meaning to a person who is from the same region. If he travels to a region in which it is not commonly used, however, he may have to explain the meaning of the phrase after he uses it or use altogether different wording to express himself.
Typically, people use the phrase "here today, gone tomorrow" to describe things they like, desire, or want more of, rather than things they are happy to see leave quickly. For example, a person may describe happiness as something that is here today, gone tomorrow, as happiness is sometimes hard to hold onto. An individual may also describe money using this phrase, as many people do not have as much of it as they would like or find that they spend it far too quickly. People are unlikely to describe such things as tests, cavities, and bills using this phrase.
Idioms such as this one are commonly used in casual speech and writing. People are usually more likely to use them when speaking to friends and family members than someone they are hoping to impress, such as a potential employer. Likewise, people may use this idiom in emails and friendly letters rather than business letters.