If you hover the mouse cursor over certain parts of the Xcas window, a temporary window will appear with information about the part. For example, if you move the mouse cursor over the status line, you will get a message saying Current CAS status. Click to modify.
If you type a function name into the Xcas command line, a similar temporary window will appear with information about the function.
If you hit the F12 button, you will be given a window in which you can use to search the html version of the manual. If you type a string in the search area, you will be given a list of help topics that contain the string. If you choose a topic and click View, your web browser will show the appropriate page of the manual.
You can also get HTML help with the menu entry Help▸Find word in HTML help.
If you click on the ? button on the status line you will get the help index.
The help index is a list of the giac function and variable names. Along with the list, the help index window has an area listing words related to any chosen word and words synonymous to the chosen word.
You can scroll through the help index items and click on the word that you want. There is also a line in the help index window that you can use to search the index; you can enter some text and be taken to the part of the index with words beginning with that text. The ? button next to this search line will open the HTML help window.
Below the search line, there is an area which will have a description of the chosen command, and below that is an area which will have examples of the command being used. If the command is a function, then between the description and examples will be some boxes in which you can enter arguments for the command. Filling in these boxes and hitting enter will put the function on the command line.
At the top of the help index window is a Details button. If you click on that, a web page will open up in your browser with the relevant portion of the manual. If you click on the ? next to the search line, you will be taken to the HTML help window.
Besides clicking on the ? on the status line, there are other ways to get to the help index.
You can get help from Xcas by using the findhelp function. If you enter findhelp(function) (or equivalently ?function) at the command input, where function is the name of a giac function, then some notes on function will appear in the answer portion and the appropriate page of the manual will appear in your web browser.