Edward Tufte's term for it is a "word-sized graphic." A sparkline isn't built for reading exact values — it only needs to convey the rough shape: rising, falling, volatile. No axis, no ticks, no legend is the correct default, not an omission.
It means nothing alone. It needs a label (a trend of *what*), the current value, and ideally a delta right beside it — that's what completes the context the line is pointing at. Mark only the last point in the accent color, as "you are here."
Don't add a tooltip or let people zoom into it — the moment you do, it's not a sparkline anymore, it's a small line chart. If the values need close reading, just use a line chart from the start.
When to use
Use it in a table row or beside a stat-tile number, wherever you need "recent trend" as a side note, not the main event.