Great question! 
This is an ongoing process. The more you research, the more you understand. But my go-to techniques are case studies and online communities. Case studies with your customers help you to access the language they use to describe their problems, as well as the language they use to describe what success looks like to them. In the case of online communities, when customers talk to their peers, they are either providing feedback to others (aka trying to help) or they are being vulnerable and are asking for advice.
Either way, they are using words that reflect how they perceive their challenges and what they think is a good approach. They are looking for some sort of validation. The language they use in these conversations can unlock incredible insights for marketing messaging.