This means that updated clients will reject connections with nodes running the old active protocol (70917).
At block B=2153200 (roughly in 24 hrs) the new consensus rules (time protocol, masternode signatures, blocks version 7) will be enforced. Clients not following the new rules will fork.
180 blocks before B (thus at block 2153020) SPORK_8 (MATERNODE_ENFORCEMENT) will be temporarily disabled. Blocks without masternode payout will be considered valid by the network.
101 blocks after B (thus at block 2153301) SPORK_8 (MATERNODE_ENFORCEMENT) will be re-enabled.
201 blocks after B (thus at block 2153401) SPORK_17 (COLD_STAKING) and SPORK_18 (V4 SPENDS) will be enabled for the first time. From that block onwards it will be possible to create coldstaking delegations and spend v1 zerocoins.
Difficulty: Due to the new time protocol, after block B there will be a massive drop in the reported difficulty on the network (likely by a factor 15-20x). This is normal and does not imply an equal drop in total staked coins but rather a different representation of the stake weight. It is due to the restrictions set by the new time protocol over the proof of stake algorithm.