Matched based on MMR, not based on rank. If ranks are different, it means the higher ranked people are really bad for their rank and actually play at around the lower ranks skill level, or the lower rank players are really good for their rank and are playing at the higher ranks skill level, or some combination of the two.
If you haven't played that many ranked games (like under 100), odds are your mmr hasn't necessarily fallen/risen to your 'true' skill level, so that can complicate things if you're being based on skill you don't actually have. Similar applies to any teammates/opponents who haven't played many ranked games.
Most people new to ranked start around silver 4 ish, so theres going to be a lot of those not-enough-games-played people around. In many cases, those people are destined to fall down, rather than climb up, as all their prior experience is in normals where people aren't necessarily trying very hard to win and/or will be playing off roles/champs, so the contrast of facing 5 people on their best champs can be a sobering experience at first.