First, it only works for Sgt. Gideon, the Green Marine with Range 0. Normally, when you have to defend (GS Attack Phase), you need to roll over the number of GS in the swarm attacking to survive. So:
1GS: 2+ roll (66%)
2GS: 3+ roll (that's 50-50)
3GS: 4+ roll (33%)
4GS: 5+ roll (17%)
5+GS: auto-kill (Artifact can save you)
With Block, you need to roll EITHER like above OR just any skull result (if you didn't roll higher). So 1GS swarm, you have 83% survival rate (anything but a 0 keeps him alive). And it turns those 5+ GS swarms that are auto-kills, into 50-50 survives (since there is a 1S, 2S and 3S on the die). Block, being a Support Action also allows you to place a Support token, which can be used to reroll the defense roll for Block, further increasing the odds of survival. And Block works against attacks from behind Gideon as well (unlike Yellow team's Support Action). He can't of course use a Support token to reroll an attack coming from behind him, but still, regardless of the swarm size, he always has at least 50% chance of survival.