First, so you can have the passages right here, I'll type them out for you (NIV translation). For background, both passages come after the parable of the sower and the seed, in which "the seed is the word of God" and the various kinds of soil describe the various types of people that hear and process that Word.
Mark 4:21-25
"[Jesus] said to them, 'Do you bring in a lamp to put it under a bowl or a bed? Instead, don't you put it on its stand? For whatever is hidden is meant to be disclosed, and whatever is concealed is meant to brought out into the open. If anyone has ears to hear, let him hear. Consider carefully what you hear,' he continued. 'With the measure you use it will be measured to you- and even more. Whoever has will be given more; whoever does not have, even what he has will be taken from him.'"
Luke 8:16-18
"'No one lights a lamp and hides it in a jar or puts it under a bed. Instead, he puts it on a stand so that those who come in can see the light. For there is nothing hidden that will not be disclosed, and nothing concealed that will not be known or brought into the open. Therefore, consider carefully how you listen. Whoever has will be given more; whoever does not have, even what he thinks he has will be taken from him.'"
(Anything quoted below without a specified reference comes from one of the two passages above.)
Here in these passages of scripture, the light of the lamp represents Jesus and the new capacity for knowledge/wisdom that was not possible before he came to earth.
When Jesus came, mercy and grace allowed humans to begin to cultivate a relationship with God on a personal basis. That event revealed various parts of God's nature and will, and these became more clear to those who know and pursue him. In Isaiah, in the same section but shortly after the prophecy that predicts John the Baptist, it says that "the glory of the Lord will be revealed, and all mankind together will see it."
In light of this, the second part of the passage becomes more clear: "nothing is hidden that will not be disclosed" because God would not send his Son with a new revelation of himself, and then pursue a relationship of intimacy while withholding himself from us. (There are aspects of God that are too great for us to know right now, that's part of God's mystery, but these will be known eventually, for we will see God in all of his splendor when we get to heaven.)
Although we know there are things we cannot know yet, there ARE aspects of himself that God has revealed or is in the process of revealing. This is why we are commanded to "love the Lord your God with all your heart and with all your soul and with all your mind" (Matthew 22:37). All of these aspects let us discern, embrace, and implement the words that God does give us.
With all of this in mind, I believe that the command to "consider carefully how you listen" and "consider carefully what you hear" is a reminder of how important it is to listen to the word of God at all times. Also it is important to note HOW we proceed to listen, and if we do hear, it is also important to consider WHAT we are told. In this way, we can know if we are people receiving the seed onto healthy soil or unhealthy soil, as described in the parable. If we make efforts in this area, those things that God would choose to reveal to us are sure to be brought into the light!
The last part of the section deals with measures. Matthew 7:1-2 states "Do not judge, or you too will be judged. For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you". Here, 'measure' describes the judgment that precedes, and in the above passages, I would assert that 'measure' applies to hearing and listening. I'm not exactly sure how they apply as it seems slightly ambiguous. It is possible that these are describing the relationship between listening correctly to the word and the increase in knowledge/understanding. It's also possible that if our capacity for listening increases, mutual communication (God to us, and us to God) is improved.
Again, this is merely my interpretation of the passage. This accounts for what the passage means for me and is not necessarily 'right'. Thank you for reading! Happy wonderings, friends.
