bottom line is that i want to read analog input that ranges from -5 to 5 (might be -10 to 10) and i'm not sure what kind of voltage divider to use ,if any .
You must use a circuit to scale the voltage to a range Teensy can accept.
Here's the simplest possible circuit. There's also a link on that page to a conversation about better but more complex circuits.
https://www.dorkbotpdx.org/blog/paul/control_voltage_cv_to_analog_input_pin
could you explain what is the ADC reference and when should i use it?
The reference is how the ADC "knows" what voltage is. Internally it compares your signal to the known reference voltage.
If the reference voltage changes, so will the result. For reading pots, where you signal is the power divided by resistors, you *want* the ADC to scale its results if the power changes.
But for reading an external voltage, you want to the reference voltage to remain stable, even if the power supply changes. So for this sort of use, you would want to select the internal 1.2V reference. Or if you need extreme accuracy, you might buy a really high quality reference and connect it to the AREF pin. But the 1.2V internal one is pretty good and very easy to use, so go with that first.
also about analog ground , which is related in a way i can wrap my head around ...
If in doubt, just connect everything to GND and don't use AGND at all. For many projects, it makes no difference.
The sort of case where it does matter is usually when you have a PWM pin controlling a motor or LEDs, or you use a servo motor. If the analog signal changes in response to the large & unrelated power consumption, then the AGND pin might help.
or can i change it to 5v or even higher ?
NO. If you try connecting 5V to AREF or any pin on Teensy 3.6 other than VIN, you will very likely destroy the board. Don't do that!
The 1.2V internal reference works very nicely if you use a circuit to scale the voltage, like the one I linked above.