For phones that are dual SIM, they will have 2 IMEI numbers whether there are actually 2 SIM's installed or not. The IMEI number is a unique number that identifies each SIM slot.
The ICCID number is the identification number for the SIM card that is installed. No 2 SIM cards will have the same number.
The SEID number stands for Secure Element and used to identify the NFC chip used for Apple Pay or other near-field communication.
It is likely you will never need to know any of these numbers, but if you are talking to your carrier and they want to know the ICCID number of your SIM card or the IMEI number for the slot it is installed, you will know where to get it.