The chances are if you are doing any kind of exercise, like cycling, even gently, you will sweat. If this water vapour can’t escape, then it will condense on the inside of your waterproof jacket, it won’t take your body heat with it, and you’ll end up with that ‘boil-in-the-bag’ feeling.
Breathability is the measure of how permeable a jacket is this water vapour, and it’s one of the main selling points of mid-to-high range waterproof jackets.
Things that are completely waterproof, for example plastic, are also not breathable. You can have an entry-level jacket that’s really waterproof, but won’t be very breathable.
This is why as you go up in price you start to get expensive waterproof and breathable membranes, like GoreTex and eVent. These work in various ways, but essentially have very tiny pores in the fabric – small enough to prevent large water droplets getting through, but allow water vapour from your body to escape.
Breathability is measured in the somewhat complicated grams of water vapour that can pass through a square meter of fabric in a 24hour period, or g/m2/d, or – as you’ll mostly likely see it written – g. Much shorter. Again, jackets range between 500g and 2,000g.
There are other elements that can help keep you dry and cool when riding, and are used a lot in entry-level jackets, and they include vents.