You can assign a note to each cell by choosing Insert - Note. The note is indicated by a small red square, the note 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 Note from the context menu of the cell. Doing so keeps the note visible until you deactivate the Show Note command from the same context menu.
To edit a permanently visible note, just click in it. If you delete the entire text of the note, the note itself is deleted.
Move or resize each note as you like.
Format each note by specifying background color, transparency, border style, and text alignment. Choose the commands from the context menu of the note.
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.