Why length used to cost you speed
On Thunderbolt 3, a longer lead meant slower data unless you bought a different kind of cable. Eaton's explainer sets out the old rule: "A passive Thunderbolt 3 cable can only transmit at the full 40 Gb/s data rate over a distance of 0.5 m (18 inches)." To hold that speed further out, "for the maximum 40 Gb/s throughput up to 2 meters, you'll need an active Thunderbolt 3 cable." So a passive 2m Thunderbolt 3 lead didn't fail outright. It ran at half speed.
What Thunderbolt 4 changed
Thunderbolt 4 made length a non-issue up to 2 metres. Intel, which owns the standard, says "Thunderbolt 4 ports have a 40Gbps (gigabits per second) bidirectional bandwidth", and a Thunderbolt 4 cable up to 2m carries all of it. You no longer need a bandwidth-versus-length chart, or to work out whether a short cable is passive or active. Check that it's a Thunderbolt 4 cable and no longer than 2m.
Passive, active, and when the difference bites
A passive cable is a plain set of wires. An active one has electronics built in to keep the signal clean over distance: in Plugable's words, "specialized Intel circuits, called retimers, along the data path that act as relay stations to boost the signal". That's what buys the extra length, and you only need it past 2 metres.
How long a cable do you actually need
If the run from your laptop to its dock, monitor, stand or shelf is 2 metres or less, a 1m or 2m Thunderbolt 4 cable covers it. You only need an active cable when the gear is genuinely far apart: a dock across the room, a display wall, or a lead routed through a desk grommet and along a cable tray. Measure the run before you buy.
When you buy, the Thunderbolt 4 cable guide sticks to certified leads. If you can't yet say whether your ports are Thunderbolt or ordinary USB-C, settle that first with the USB4 vs Thunderbolt 4 explainer: the length rules here only govern a Thunderbolt 4 link. And if the long cable is for a dock parked out of the way, the USB-C docking station guide covers docks built to sit at the end of a single lead.
FAQ
Q: Will an old active Thunderbolt 3 cable work for everything? A: Not quite. Plugable points out that "you can get an active Thunderbolt 3 cable, but it only transmits Thunderbolt data, not USB," so it can leave an ordinary USB device dead. Its advice for a long lead: "If you need an active cable, stick with Thunderbolt 4, it's more versatile."