Undo and Redo - The first two options will undo and redo respectively.
Alternatively, you can use keyboard shortcuts Ctrl + Z and Ctrl + Y for undo and redo respectively.
You can use the following options for selected text, or you can set the options first, and its effect will be applied to the text you're going to write.
Font - This one is used to change the font. Hit it and then pick the one you prefer.
Alternatively, you can press Ctrl + Shift + 5 to select the previous font and Ctrl + Shift + 6 to select the next font in the font list.
Size - This option is for changing text size. Hit it and then choose the one that you prefer.
Alternatively, you can use the keyboard shortcuts Ctrl + Shift + - (used to decrease text size) and Ctrl + Shift + + (used to increase text size).
B - For bold text (or press Ctrl + B).
I - For italicized text (or press Ctrl + I).
U - For underlined text (or press Ctrl + U).
A - This option is to change text and background color. Hit this option and then:
The one you chose under the "Text color" section will be your text color.
The one you chose under the "Background color" section will be your text's background color.
Align - This option is to align text. Hit this option and then choose Align left, Align center or Align right. Alternatively, you can use the shortcuts - Ctrl + Shift + L (for left align) or E (for center align) or R (for right align).
"Numbered list" (shortcut - Ctrl + Shift + 7) and "Bulleted list" (shortcut - Ctrl + Shift + 8) options are for numbered and bulleted list.
"Indent less" and "Indent more" options are used to decrease and increase intent.
Quote - To quote the selected text (or press Ctrl + Shift + 9).
S - To strikethrough text (or press Alt + Shift + 5).
The last option in the toolbar is for removing formatting. Select the text for which you want to remove formatting and then hit this option (or press Ctrl and \).