what is reset generator ?

Generally, any circuit which generates a RESET pulse for microcontroller/cpu.

The datasheets usually give RC circuit as reference, which has many short comings. The RC circuit is the simplest form of a circuit which can generate reset signal.
The more accurate and reliable method for generating a reset pulse is to use a reset generator chip, specifically designed for a guaranteed reset signal.
A reset generator monitors the supply voltage of the system and generates a reset again if the supply falls down below a specified level (known as brown out detection).
Some chips even take an input (named strobe or some other name) from the micro/cpu, which they monitor for a periodic change. If there is not change in the strobe signal (for a predetermined time) then the reset generator assumes that micro/cpu is not working (there may be lot of reasons for this), then reset signal is generated again (this action known as Watchdog timer).

Now the RC circuit has the short-coming of brown out detection, watch dog timer. You can't be sure of if an RC will always generate a good reset signal.

Thats why, a reset generator chip is recommended.