Every headphone specification sheet carries an impedance figure in ohms and a sensitivity figure in decibels. Between them they answer one question (will this pair go loud enough, cleanly, from the thing you are going to plug it into) and almost nothing else. They are routinely read as though they said something about sound quality. They do not.
Impedance is resistance to current
Impedance is how strongly the headphone opposes the alternating current an amplifier pushes through it. It is quoted as a nominal figure, usually 16, 32, 80, 250 or 300 ohms, and it varies with frequency. The nominal number is a convenient single value, not a constant.
Low impedance means the headphone draws more current for a given voltage. That is why 16 and 32 ohm headphones get loud from a phone: a phone can supply a bit of current but not much voltage. High impedance means the opposite. A 300 ohm headphone needs a lot more voltage swing to reach the same level, and a small source hits its voltage limit and starts clipping before it gets there.
Sensitivity is loudness per unit of input
Sensitivity is quoted either as decibels per milliwatt or decibels per volt, and the two are not interchangeable. dB/mW tells you how loud the headphone goes for a given amount of power; dB/V tells you how loud it goes for a given voltage. If a spec sheet does not say which, it is usually dB/mW, but you cannot compare two figures unless you know they are the same unit.
This is the figure most people should be reading first. A 300 ohm headphone with high sensitivity can be easier to drive than a 32 ohm headphone with low sensitivity, which is exactly the case where reading impedance alone gives you the wrong answer.
Reading them together
What you want to know is whether your source can deliver enough voltage and current to reach a comfortable listening level with headroom left over. Headroom matters because music is not a steady tone: a track that averages a comfortable level has peaks ten or fifteen decibels above it, and those peaks are what clip first.
As a working rule, above about 100 ohms or below about 95 dB/mW, plan on an amplifier. Between 16 and 60 ohms with sensitivity above 100 dB/mW, a phone or a laptop will usually be fine. In the middle, it depends on how loud you listen and how dynamic your music is.
What neither number tells you
Neither figure predicts tonal balance, detail, soundstage or comfort. A headphone that is easy to drive is not a better headphone; it is a headphone that works with more sources. Two pairs with identical impedance and sensitivity can sound completely different, because that is decided by the driver, the enclosure and the pads.
Use impedance and sensitivity to rule out combinations that will not work. Then choose between what is left on enclosure, fit and how it sounds to you.