If you look at the pictured inside view of the 8-track tape's endless loop, you see that each tape contained its own rubber idle pinch roller which made contact with the motorized capstan of the tape player when the tape was fully inserted. Thereby providing the friction to move the tape across the playback heads.

Across the width of the tape, there were actually 4 choices for program material you could select as you were listening, therefore 8 individual "tracks" had been recorded across the length of tape in the same direction, in stereo configuration to provide both left & right channel sound for the 4 choices.

Common players would have a single stereo pickup head that would physically move to a different position across the width of the tape when you changed "channels" between the 4 target positions, picking up 2 tracks of stereo material and ignoring the other 6 tracks on the tape as they pass.

Anyway, up the chart is the far less common predecessor, the 4-track tape. Almost the same cartridge as the 8-track turned out to conform to, except there were only 4 tracks and therefore only two channels to choose from at any one time. And this earlier iteration did not have a pinch roller inside each tape, instead there was a big hole in the bottom of the cartridge where the pinch roller of the tape player itself would mechanically move into position inside the tape to get it going.

After all, reel-to-reel players had always had a single pinch roller per player. It's not a big leap, but the required actual paradigm shift to the 8-track format made all the difference in reliability.

And left the 4-track in the dust.