The question is too general, you need to refer at least to used memory technology (DDR, DDR2, DDR3, DDR4). They have different termination options and due to different speed, different requirements. Newer DDR families have mostly dynamic on-chip termination (ODT) and no termination resistors. Fixed resistor termination has been used with DDR and partly DDR2.
If the driver sends out a signal. It travels along to the end.
* without proper termination it becomes reflected and the reflection signal travels back to the driver. Causing refelction error all the line.
* with end_of_line termination .. there is no reflection. No erroneous echo signal travleing back.