Dave has some good info too, but here's some background.

Transmitters ALL have noise coming out that decreases as you move away from
the carrier. We call this sideband noise. This noise can cover up your
received signal if it is high enough (which is not difficult) on the receive
frequency. A duplexer or cavity set can (if needed) have a notch at the
_RX_ frequency to reduce this noise. Cavities like this are called
"pass-reject" cavities because they have a band-pass characteristic for the
transmitter frequency and a notch which can also be tuned to the receiver
frequency to reduce this noise more than the simple cavity skirts can.

Transmitters can have spurs and if you have that, you have a similar problem
because it is just a single frequency which could cover a received signal
and you must reduce. However, chances of having a spur on your receiver
frequency is rare (though synthesizers can cause quite a few near the
carrier, so you want to stay away from a synthesized radio if possible, for
a repeater, for that reason)

While we talk of "receiver overload, blocking, desense" receivers have one
characteristic which is not obvious to most. Since ALL oscillators have
this sideband noise mentioned above, the local oscillator IN the receiver
also has some. When a strong carrier enters the receiver (your own
transmitter), _it_ becomes another local oscillator in the mixer and can mix
with the sideband noise of the intentional LO. This mixes some part of the
sideband noise of the LO onto the desired frequency. This is one of the
mechanisims for receiver desense and one reason to reduce the amount of TX
power getting to the receiver. With synthesixed receivers, it CAN be the
dominant cause for desense. This is because the VCOs needed for
synthesizers typically can have quite high sideband noise. Since a typical
radio is not designed for repeater operation, it's design will not try to
optimize for sideband noise.

On 2M you can probably hear this effect with just about any two 2M ham rigs.
Talk to someone on the local repeater, but not too close to the repeater
(I'd say at least 5 miles away) and drive close to each other. At some
point he repeater will get noisy and if you have a good ear, you can hear
that this noise is different than the normal, weak signal noise. When the
other station de-keys, the repeater will quiet with it's normal gusto.

73, Steve, K9DCI