That it lives within your comfort level of risk/reward, and that you understand the downsides and the upsides. Generally, the more risky something is, the better upside. Very risk adverse? Look to something like Bonds, but the upside is lower. If you want to be comfortable, you need to factor in your timeline too. If you want your fancy new car next year, you have to steer less risky, while if you are looking at a 20-30 year timeline, a dip this fall or next year won't matter too much. And you also want a bit of diversity -- if you invest broadly, things that are going up are balanced with things that aren't doing as well.
Over a number of years, you can do very well by sticking with such a strategy. And, being comfortable means you are more likely to stick with it, rather then sell if something doesn't go great.