A reel strip is the complete ordered list of symbols on a single reel. You only see a window of it, usually three or four positions, but the strip may be much longer.
The make-up of each strip is one of the main tools in a slot's maths. Putting more low symbols and fewer high symbols on a strip, or placing wilds only on certain reels, changes how often combinations land and so shapes the RTP and volatility.
Games often use different strips for the base game and the free spins round, which is why features can feel more generous: they are running on different reels. Some games with several RTP versions achieve the difference partly by adjusting strips.
On physical machines the strip was printed on the drum. The virtual reel idea separated the weighting of a stop from its physical appearance, and online games carry that further with weighted outcome tables.