Exactly what it says. The users interface.
Not to be confused with user experience. That is how they feel about your product or service.
The interface is how they interact with it. How easy and seamless is it to use? You can have a great user experience that alleviates a pain point and a terrible user interface.
This is where you get close with your target market. Get out there and talk to them. Watch them interact with your products.
Most importantly, for us at least, get them to complain about it.
Find out those annoying little quirks in your software that they tolerate because they need the pain alleviated but they wish was different.
It can be hard though. If your alleviating enough pain people will overlook the little stuff.
Keep digging though. This is a continuous process that should constantly be looked at and evaluated.
This part will never be done. It might be close to perfect but perfection should be like an aspiration that you want but know that the closer it gets the further away it really becomes.
Sometimes you need an outside set of fresh eyes to help out with this. Thats what we are here for.