The block 5 Falcon 9 will have higher thrust on all of the engines and improvements on landing legs compared to the block 4 Falcon 9 Full Thrust. There are also a number of small changes to streamline recovery and re-usability of first-stage boosters. Alterations to the launch vehicle are primarily focused on increasing the speed of production and efficiency of re-usability. SpaceX aims to fly each Block 5 first stage ten times with only inspections in between, and up to 100 times with refurbishment.
Block 5 changes are mainly driven by upgrades needed for the Commercial Crew program and National Security Space Launch requirements. They include performance upgrades, manufacturing improvements, and "probably 100 or so changes" to increase the margin for demanding customers.