vSphere FT provides provides zero downtime and zero data loss by creating a duplicate (secondary) copy of the VM on a different host and keeping the two VMs in sync. The secondary VM immediately takes over if the primary VM fails.
The technology that ensures that the primary and the secondary VM stay synchronized and enables the workload to run on two ESXi hosts simultaneously is called VMware vLockstep. It works by placing one virtual machine (the primary) in the active state. This VM is receiving requests, responding to these requests, and running applications. Another VM (the secondary) acts as a silent partner. If the primary VM fails, the secondary VM will immediately take over. A new secondary VM will be created on a different host. Here is an illustration of the process (image source: VMware):
In the picture above you can see that the host on which the primary VM was running failed. The secondary VM becomes the new primary VM and a new secondary VM is created on another host.