Staff and system text
For general-purpose text, use Staff Text or System Text. The difference between these two types of text is whether you want it to apply to a single staff, or the whole system. This matters when extracting parts: staff text will only appear in a part that contains the specific instrument the text is attached to, while system text will appear in all parts. Additionally, if you choose to hide empty staves, any staff text belonging to an empty staff will also be hidden. System text is never hidden by the "hide empty staves" feature.
Staff text
Staff text is general purpose text associated with a particular staff at a particular location in the score. To create staff text, choose a location by selecting a note or rest and then use the menu option Add → Text → Staff Text, or use the shortcut Ctrl+T (Mac: ⌘+T). A small text box appears and you can immediately start typing. You can exit the text box at any time (even without typing anything) by pressing Esc.
Staff text can, for example, be used to apply indications such as "Solo" or "Pizzicato" to one staff in a score. Depending on what the instructions of the staff text are, MIDI playback of that staff at the text location can be altered to match the instructions by right-clicking on the staff text and selecting Staff Text Properties…. See Mid-staff sound change.
System text
System text is used when you wish to apply text indications to a whole system rather than just to one staff line. This makes a difference when extracting parts, or if you choose to hide empty staves. To create system text, chose a location by selecting a note or rest and then use the menu option Add → Text → System Text, or use the shortcut Ctrl+Shift+T (Mac: ⌘+Shift+T). A small text box appears and you can immediately start typing. You can exit the text box at any time (even without typing anything) by pressing Esc.