In those days (near the end of the 16th century) music used modes instead of the modern system of major and minor keys. Sometimes the music would not sound right unless a note was sharpened or flattened (raised or lowered a semitone). For example, to go down from the note B to an F would have sounded bad, so the F would need to be an F sharp. The composer who wrote the music down might not bother to put the sign for F sharp. He would expect the performer to know that it should be an F sharp and not an F. This is what is called musica ficta.