Health in Chronicle X is a stat of Models, with a starting value given by their stat card or stat board.
For Heroes, the Health is given by the maximum value of their Health Dial on their Hero Dials Board. It is normally 8. When a Hero receives damage, the Health Dial is turned to a lower value, which also causes two of the other stats the Hero has to change.
For Alien Units, the Health is tracked by putting Wound tokens next to the Unit on the Mission Map; or alternatively, if there is only one such Alien Unit in play, the tokens can be put on the Alien Unit card instead. Some Units can also lose Health by losing members of the Unit.