Lord
Background
A lord can send reinformcents to his/her vassals when ever they are under attack. A lord typically acts as a `big-brother` or `big-sister` helping other vassals under him/her.
Independance
You can gain independance from your current lord by spending a certain amount of honors. For more information on honors please check Diplomacy > Honors. First of all there is no harm being someone`s vassal and to have a lord. Having a lord is similar to having a `big-brother` or `big-sister` to help you out in the game. It is only recommended that you declare independance only if you wish to change lords. This is applicable if your lord is not helping you out as much as he/she should. ALso so long as you are not independance you can not change lord therefore the reason for gaining independance comes along.
Tips & advise for lords
Since your empire level depends mainly on vassals, it is only logical that you should help your vassals out. You can help your vassals by building ships in your outpost so your vassals will have some form of reinforcements when they are attacked. You can also help your vassals our by teching them some tricks or strategies in order for them to play better in the game. Ultimately the as your vassals improves in regards to their planet and population size the more your empire level improves as well