What is the meaning of this symbol? The line between the cars.

 

See callout 🄳 on the illustration on page 197 of the 2024 LEXUS ES300H OWNER'S MANUAL USA AND CANADA ONLY. And from page 198:
Stated another way, when the vehicle icon and the center line are displayed while Lane Trace Assist and Steering Assist are on, your ES's steering assist is using the vehicle in front of you to set the path steering assist will make your ES follow. When the center line and vehicle icon aren't displayed while Lane Trace Assist and Steering Assist are on, your ES's steering assist will use the lane markings (and, if needed, other lane or road boundary visual indications) to set the path for center-of-lane travel.

 

The horizontal lines indicate the distance of "closeness" between your vehicle and the one in front of you. There are 3 level of distance you can cycle thru on your steering wheel adjustment. I usually select 1 bar which allows your vehicle to get closer to the vehicle in front of your before it automatically start braking.

 

That's the vehicle-to-vehicle distance setting display/control. From the "Vehicle-to-vehicle distance settings (vehicle-to-vehicle distance control mode)" section of the "4-5. Using the driving support systems" subchapter:

There's a table of setting-associated distances that "… correspond to a vehicle speed of 50 mph (80 km/h)."

For those who were taught and follow the 2-second follow distance rule of thumb, 50 mph is 73.3 ft/s, so the 2-second rule gives a 146.6 ft follow distance at 50 mph. The long distance setting has a follow distance of "Approximately 160 ft." - a slightly longer follow distance than the 2-second follow distance rule of thumb. The medium distance setting has a follow distance of "Approximately 130 ft." - a slightly shorter follow distance than the 2-second follow distance rule of thumb. And, the short distance setting has a follow distance of "Approximately 100 ft." - within a follow distance range when you take the 0.5 to 1-second closely attentive driver reaction time out of the appropriate following distance determination.

I too select the short vehicle-to-vehicle distance setting, but at that setting, you really need to pay attention, especially so in heavy traffic conditions. The system doesn't handle the jackrabbit lane changer very well in fast-moving, heavy traffic nor the let-me-in wedging lane changer in slow, crawling traffic. Also, in fast-moving to dead-stop traffic type patterns, when there's enough distance behind the preceding vehicle for non-panic braking, the system tends to delay slowing down or braking until you get into panic BRAKE! territory.

 

I find using the shorter distance setting reduces the number of other drivers that are jumping in front of me which automatically engages my brakes which is annoying. I’ll us the longer setting when the traffic is lighter or if there are a large number of 18 wheel trucks sharing the road. They need more space to cut in front of you.

 

What year/trim has this display? I have a ‘21 with premium package & don’t recall seeing it but I very rarely use lane centering just LTA.

 

If you don't have the lane centering subfunction of the Lane Trace Assist system turned on, you won't see the line between the cars esotof was asking about.

Every model year and model trim has the display, but there are some differences in instrument cluster layout between F Sport and non-F Sport model types. The above post #1 photo is of the F Sport instrument cluster with the main meter moved to the right with the Multi Information Display (MID) set to the driving support system information display (stylized vehicle between lane markings icon) menu icon screen.

In all versions of the Owner's Manual, the "Indications on multi-information display" in subchapter "4-5. Using the driving support systems" has the Lane Trace Assist illustration with symbol callouts and explanatory text for each callout. What callouts 🄲 and 🄳 point to are swapped between the 2019 through 2022 and 2023 and later model years versions of the Owner's Manual.