Can somebody explain what IQ (quadrature) means in terms of SDR?
The I and Q components are not the same signal; they are samples of the same signal that are taken 90 degrees out of phase, and they contain different information. It's a subtle, but important distinction.

Separating I and Q in this way allows you to measure the relative phase of the components of the signal. This is important not only for FM (and PM) demodulation, but also for any other situation in which you need to distinguish the contents of the upper and lower sidebands of the carrier (e.g., SSB).

Whenever a frequency conversion (heterodyning) occurs in an SDR (particularly in the analog front end), the I and Q components are handled differently. Two copies of the local oscillator are generated, one 90 degrees delayed with respect to the other, and these are separately mixed with I and Q. This preserves the phase relationships through the conversion.

EDIT:

All this really means is that you're sampling the signal at a high enough rate to capture all of the sideband information on both sides of the carrier. I and Q are really just a notational convention that makes the math work out a little more cleanly. It becomes most relevant if you end up heterodyning the signal directly down to baseband (synchronous detection). If you don't preserve both I and Q, the two sidebands get folded on top of each other (a form of aliasing) and you can no longer decode FM, PM or QAM signals.