This abbreviation is Latin for "let it stand." In other words, the piece of text in question is not a typographical error, although the editor had thought so at first. If you see this mark on your written work, that means the editor is giving you permission to leave that part of the text as is.
If you find this abbreviation on your work, your editor has found that you have shifted from one topic to another without a smooth transition, which has disrupted the flow of your paper. You need one or two sentences to transition from one thought or paragraph to another. The purpose of this transitional section is to show a relationship between two adjacent paragraphs. For example, if you are writing a paper on "Soul Singers of the 70s," and you are discussing two artists, Al Green and Billy Paul, who both had number-one hits in 1972, you can transition from your discussion of Green to your discussion of Paul by writing: Later that same year, another soul singer named Billy Paul went to number one with "Me and Mrs. Jones," a song about an extramarital affair.
In this instance, the editor is asking you to change the font you have used for your paper. It means that your letters are either too small or too large. You should ideally use 12 point Times or Helvetica, or something similarly legible, although 10 point is also acceptable. According to Merriam-Webster, a variant of this abbreviation is an italicized "wf."
If you find this comment, it means that your proofreader has found an instance of subject-verb disagreement. Your verb must always agree in number with your subject. In most instances, a plural noun takes a plural verb, and a singular noun needs a singular verb. For instance, it is an error to write a sentence like, "They was all ticketed for not wearing seat belts." You must replace "was" with "were," since the presence of "they" indicates that the incident involved more than one person.
This denotes a sentence fragment. A simple sentence must have a subject and a verb. When one of these elements is missing, you have only part of a sentence, or a dependent clause. To correct this situation, you link the fragment to the preceding or following sentence. For instance, "My English professor," is not a stand-alone sentence. It needs a predicate, or a verbal phrase indicating an action on his part. You may revise this sentence by saying, "My English professor failed my last paper."
This designation denotes the presence of a misspelled word. Also, it could mean that the word, although it is not intrinsically a misspelling, is the wrong member of a homonym set given the overall context of the sentence, although you have spelled it correctly. For instance, in the sentence, "Don't go to far, and don't stay to long," the writer has used the wrong member of the "to/two/too" homonym family. The correct word in this sentence is "too."