Attiny45 reset?

I am using the attiny45 for a small arduino project, but have some questions about it. There is a pin labeled 'reset,' what does this do and how do I use it?

From the datasheet:

1.1.4 RESET
Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock
is not running and provided the reset pin has not been disabled. The minimum pulse length is given in Table 21-4
on page 165. Shorter pulses are not guaranteed to generate a reset.
The reset pin can also be used as a (weak) I/O pin.

You can read more here: AtTiny45

I guess my main question is, is what does resetting it do exactly? Thanks!

Reset means reset, all registers return to default value, all pins tristated, etc. When reset is released, program starts as if device was just powered on.

The Reset pin is also used to program the chip via ISP/ICSP (during the whole ISP programming process, reset must be held low).

Reset can be disabled by setting the RSTDISBL fuse, making it into a normal IO pin (except with very limited source/sink capability), but then you can't program the chip except via HVSP.