Click on the heading of interest and press Show Topics. This produces a list of Help topics on the subject highlighted. Choose the reference you think is closest to your query, then click on the Go To button. The information provided may, for example, tell you how to perform a task, or give you reference information about the command selected. If you click, a box explaining the text in green will appear.
Help is also available for particular dialogue boxes.
If you have started an operation, but are uncertain how to proceed, the Help button will describe each of the options available to you, as well as enabling you to use the Help menu in a more general way. Pressing F1 also gives access to Help, supposedly tailored to your requirements. It is worth experimenting with the Help command to see how it works. Note that one of the options on the menu is an introductory guide to Excel called Quick Preview. You might find this useful.