You've probably heard this in terms of customer service. The guys at the service station say the car will be ready on Friday but call you Thursday morning and it costs a bit less than they originally thought. The Microsoft tech support says it'll be $35 but help you fix it for free anyway. You get the idea.... THEY set an expectation that YOU can live. Then, they provide better service for less time or cost.
The same thing happens in great relationships. Here's an example: Your wife has an important business dinner. She asks you to be home at 5:30, giving her 45 minutes to put dinner on the table, get ready and make it on time. This is an expectation you can both live with, as long as it happens.
If you show up on time - congrats, you've done what you said you'd do. No medals awarded there but none expected. If you are going to be late, you better call (and it better be from the local emergency room). If you show up with the kids' favorite pizza in hand at 5 pm and tell her "I got it - go get ready" with a smile on your face, you are over delivering.
This is what friends do - they focus on what is important to the other person and over deliver in their support. I'm not saying you under promise everything. The wedding vow is "I DO" not "I'll TRY." And, I'm not saying you should be helpful pest.
But, in the everyday situations of a busy life together, taking a moment to over deliver for your loved one shows that a) listen and b) care.