There are various ways you may refer to the system of components on a phone that accept and communicate with the SIM card. When referring to a SIM tray for repair the problem is usually with the SIM Reader.
The SIM reader is the electronic component on the main logic board which allows the phone to communicate with the SIM card.
A SIM card (A) supplied by your carrier gives your phone access to their cellular network (for phone calls) and LTE network (for data).
The SIM card is placed into the SIM tray (B) which acts a caddy that fits into the SIM Slot (C) on your phone.
Sometimes there is no actual SIM Tray involved and the card fits directly into the slot.
Once inserted into the slot the contacts on the SIM card make a connection to the pins of the SIM Reader (D).
When you see messages like:
it typically means we will need to do a repair on the SIM Card Reader located on the main logic board.