We've all worked with people who have made promises they didn't keep. It's frustrating when someone says one thing and does another, and it can slow a group's progress. You can be an asset to your team by delivering what you said you would do – on time.
Being reliable also applies to the work you do for the group. If you have high standards, people will depend on you to produce quality work. If your output is excellent one day, but only average the next, the team may regard you as being unreliable.