Take heed what ye hear - Or, consider well what you hear. Make a good improvement of it.
With what measure ye mete ... - You shall be treated according to the use you make of your opportunities of learning. If you consider it well, and make a good improvement of what you hear, you shall be well rewarded. If not, your reward shall be small. This is a proverbial expression. See it explained in the notes at Matthew 7:1-2.
Mete - Measure. With what measure ye measure.
Unto you that hear - To you who are “attentive,” and who improve what you hear.