Understanding problems. Understanding people. Understanding environments. Understanding current solutions.
Understanding whether a problem is even worth solving, or if it's even the correct problem to solve.
Understanding helps us to solve problems in better ways. And that's what design is all about.
User Research
Whether it's market research, ethnography, interviews, surveys, diary studies or card sorting, activities that help to develop an understanding of your users (how they think, how they work, their motivations and their activities) will not only help to validate concepts, but also generate new design concepts.
Testing
Usability testing, message testing, false door experiments, design prototyping, cognitive walkthroughs... There are many methods for analyzing the viability of design concepts or the efficacy of shipped products. It's not enough to gather feedback from teammates and stakeholders. To truly succeed, you have to put your designs to the test.
Analytics
Moving past the conceptual, beyond the prototypical, analytics grant understanding of the real. By analyzing usage, developing funnels/goals and continually reviewing product performance, you can ensure that your designs are having the desired impact, or determine whether or not your work is yet finished.