What does a blue seal on bills mean and what does a red seal on bills mean?

Blue seals indicate that the note is a Silver Certificate. Before silver was withdrawn from circulation, silver certificates were backed 1-for-1 with the same worth of silver metal in the Treasury. They could be exchanged at banks for silver, although few people actually did so by the mid-20th century.

Red seals indicate a special series of currency called United States Notes.

Neither one of these series is still issued. All modern currency is issued as Federal Reserve Notes.

Gold certificates with gold seals were issued until 1928, and they were redeemable for their price in gold.