In a single-hung window only one sash moves. Almost always it is the lower one, which slides up in front of a fixed upper sash. The alternative, and the norm in British period housing, is the double-hung sash, where both sashes move on their own cords and weights.
The difference is mechanical. A double-hung sash has two sets of cords, pulleys and weights, one pair for each sash, with a parting bead between them. A single hung window needs only one set, and the upper sash is fixed in its rebate. Fewer moving parts, less to go wrong, and less to restore — but also less window to work with.
Ventilation
That last point is the practical consequence. The classic use of a double-hung sash is to open both sashes a little: cool air enters at the bottom, warm stale air leaves at the top, and the room clears without a draught blowing through it. A single-hung window cannot do this, because the top sash does not move. In a room with a fireplace, a high ceiling or a condensation problem, it is a real limitation rather than a technicality.
What it means for repair
A fixed upper sash still needs looking after. The glazing to it fails at the same rate as anywhere else, its bottom rail sits against the meeting rail of the sash below and stays damp, and it is generally in worse condition than the sash that gets opened, precisely because nobody has been paying attention to it. Fixed sashes are also frequently painted shut on top of being fixed, which hides movement in the joints until the paint is stripped.
Where the original window was double-hung and the top sash has simply been screwed or painted shut over the years, it can usually be freed and brought back into use: the pocket is opened, the weights and cords are renewed, and the parting bead is replaced. That is a common part of a restoration, and it is worth asking about, because a window that was built to work in both halves is rarely single-hung by design.