Mark 4:21–25
21 And he said to them, “Is a lamp brought in to be put under a basket, or under a bed, and not on a stand? 22 For nothing is hidden except to be made manifest; nor is anything secret except to come to light. 23 If anyone has ears to hear, let him hear.” 24 And he said to them, “Pay attention to what you hear: with the measure you use, it will be measured to you, and still more will be added to you. 25 For to the one who has, more will be given, and from the one who has not, even what he has will be taken away.”
Mark continues with his section on Jesus teaching with a rather cryptic short teaching of Jesus. This teaching can be found in various places in Matthew and Luke but Mark here continues it as a way of furthering Jesus' teaching on the parables.
Grammatically in Greek the lamp is the subject of the sentence and it is articular. The ESV (above) translates it literally. The NIV translates it, "Do you bring in a lamp..." putting lamp in the objective case for easier English reading but losing some of the subtle implications. So Jesus is emphasizing the lamp and personifying it. Jesus is then implying with this teaching that he, himself, is the lamp who sheds light on everything hidden. Jesus, his teaching, his identity seem to be hidden and secret now but they are coming to light for those who are good soil (v. 20) and have ears to hear.
Verse 24 is a version of multiple, common, Hebrew proverbs warning people to be wary of the standards they set for others because they too will be measured by those same standards. Another common proverb read, "In the pot in which you cook for others you will be cooked." The implication here is that God is the one who will measure out to you what you give out. So in this context, to those who hear and understand the parables and teachings of Jesus, God will add to them more knowledge and understanding of Jesus and revelation of the kingdom.
This parable elaborates further on the previous parable of the sower and the seed. Whoever is the good soil who hears the word, accepts it and bears fruit will be given the secret of the kingdom of God and God will continue to add to their understanding (compare v. 11 with v. 25). To those on the outside, the other 3 types of soils, even what little understanding they had will be taken from them.