It is well known that Ernest Hemingway avoided adverbs. His minimalist style didn't have room for superfluous words. Steven King said, "I believe the road to hell is paved with adverbs, and I will shout it from the rooftops," in his book 'On Writing'.
Adverbs tend to be unnecessary. They can be redundant, slow down the action, or amplify verbs that don't need to be amplified. You can often replace an adverb and the verb it modifies with a stronger verb or, if the verb is strong enough to stand alone anyway, remove the adverb and leave the verb intact.
Further reading: How to Eliminate Adverbs
These words rarely add anything of value to your writing. They inflate the word count at the cost of readability. You can make your writing leaner and more precise by omitting as filler words when possible.
You could add more words to this list. Also, not every occurrence of a word on this list is going to need to be removed either. This check is a quick reference. Start by considering whether you need these words and apply that mentality throughout your writing.
Further reading: Style at UNC Writing Center (read the section on wordiness)
These words can be indicators of passive voice. Passive voice occurs when you make the object of action into the subject of a sentence.
Passive voice is not always bad. However, it can affect the clarity or weaken the impact of a sentence. It can often be replaced with an active sentence structure to improve the writing.
Further reading: Passive Voice at UNC Writing Center
A lexical illusion is a duplicate word our brains filter out when going from one line to the next. This makes catching these issues when reading or editing difficult. This check highlights any duplicate words to allow you to check that they are correct.
These words and phrases are often misused. The list includes homonyms that get mixed up and other words and phrases that get used incorrectly. Double check them to make sure you used the correct word or phrase in each case.
Further reading: Common Misused Words and Phrases at Weber State University
This test highlights pronouns in your writing. This allows you to check the pronouns for clarity. A pronoun should refer to a specific noun, be clear which noun it is referring to, and agree in number and in person with the noun it refers to.
Further reading: Making Clear Pronoun References at Mount Saint Vincent University
The purpose of this website is to highlight potential pitfalls in your writing and encourage good habits. Few of the things this tool highlights are hard and fast rules. Try to look at the output as a chance to think about your writing choices and see if there are areas for improvement.
This website is not infallible. It will miss things and there will be false positives in the results from time to time. This is not a replacement for careful editing. That said, I do hope this provides useful feedback to you.
The article 3 shell scripts to improve your writing, or "My Ph.D. advisor rewrote himself in bash." by Matt Might inspired this website. I have expanded on it to include other things I thought would be helpful.