In Bible days, official documents were often written on parchment and rolled up as scrolls. These scrolls were sometimes sealed with wax so they could not be read (just as we lick an envelope to seal it). Once sealed, only an authorized person could break the seal and open the scroll.
This is exactly what we see happening in Revelation 5 and 6. These scrolls are not official documents however, they are revelations from God of events which are yet to take place. The symbolism of the scrolls shows us that these are things that God determined would happen but did not want revealed until a certain time in human history. When that time arrives, the seals are said to be broken and the contents of the scrolls revealed.