The undo and redo in LibreOffice

Most programmes offer an undo and redo operation. In LibreOffice including Writer and Calc, the undo will revert your last action. For example, if you have deleted a word accidentally then undo will bring it back. This also works for the previous action and action before that.
You can press Ctrl+Z or to the menu Edit and click the first entry Undo.
There is a specific way for undoing the undo-command called redo. A special command is required because using undo will go further and further back in the history of your actions. In contrast, redo works in the other direction. For example, you deleted a word and used the undo command. Now, the redo command will delete this word again, i.e. undoing the undo.
You can press Ctrl+Y or to the menu Edit and click the second entry Redo.

Other issues

In LibreOffice, undo and redo are bound to a particular document. That means clicking undo affects the active document, but not any other documents in the background. Limit in number of steps
There is a limit which is typically set to 100. That means you can undo the last 100 actions. In LibreOffice go to the menu Tools → Options... → LibreOffice → Memory. In the top right, you can adjust the number of steps that will be available to you.