You can assign a comment to each cell by choosing Insert - Comment. The comment is indicated by a small red square, the comment indicator, in the cell.
The comment is visible whenever the mouse pointer is over the cell.
When you select the cell, you can choose Show Comment from the context menu of the cell. Doing so keeps the comment visible until you deactivate the Show Comment command from the same context menu.
To edit a permanently visible comment, just click in it. If you delete the entire text of the comment, the comment itself is deleted.
Move or resize each comment as you like.
Format each comment by specifying background colour, transparency, border style, and text alignment. Choose the commands from the context menu of the comment.
To show or hide the comment indicator, choose LibreOffice - PreferencesTools - Options - LibreOffice Calc - View and mark or unmark the Comment indicator check box.
To display a help tip for a selected cell, use Data - Validity - Input Help.