What does Frontier, Homestead, Metropolis, Serenity mean?
These are the four planned stages in the development of Ethereum. We are currently in the Homestead phase, with the Metropolis fork partially completed in October 2017 with the Byzatium hard fork. The second half of the Metropolis stage is to come in the Constantinople hard fork, which was originally planned for late in 2017, but has been delayed.
There is also a Caspar hard fork being planned, which os part of the Serenity stage, and will introduce sharding and begin the transition of Ethereum to a hybrid PoW/PoS coin.