Transition words and phrases are signposts to indicate the direction of your work.  They help with the flow and indicate to the reader that you are moving from one idea to another.  This is a nice infographic explaining and listing transition words and phrases. 

Source: www.grammarcheck.net Copyright © 2021 GrammarCheck.net (Code is provided to embed infographics in other sites and therefore they can be used on other sites)

 

Be aware that most online grammar checkers are no better than the ones you see in word processing software and should not be relied upon.