The RSS shows every submission which gets at least one up vote. So most spam doesn't make it to the RSS, but a lot of other cruft can pass through depending on if at least one person liked it.
I disagree here Shamiq, from my experience that's not entirely accurate. Typically 4-5 votes makes front page - though yo uare correct, often 2-3 votes is sufficient. Sometimes 2 votes will, but not always - it's more a function (I believe) of how much front page upvoting is going on at the time.
At least, that's how I've seen it play out.