The plastic ring that protects the wires when they pass through the panel board wall. What is it’s name?
Grommet
Bushing…
While a bushing or a grommet would prevent chafing damage to the cable they are not securing the cable to the enclosure as required. There should be a cable clamp installed to hold the cable to the panel.
Connector or strain relief
I though bushing was for threaded and grommet for non threaded.
I agree with Jim a connector in each panel is required. I am curious about the black conductor with the yellow stripe.
In my area, they are called “box connectors”…appears to be one or 2 missing in your picture.
Tracer
Would you define that a little more.
The NEC uses the word stripe to describe that yellow marking on the black conductor. The word tracer is often used as well.
I raised the question as to what type of conductor that is. (I couldn’t tell definitively from the photo). From what I’ve seen this may be type USE conductors, used for underground service entrance laterals, and having a yellow stripe. Type USE condcutors are not permitted to enter into a building and must remain solely on the outside.
Some further information:
And a Bushing or Grommet, is the actual answer to the question. 
But I was educated as to the name of the clamp/connector needed in the panel, ty . 