When defining a vpc, there is the maxAzs option set to 3, I’m not sure I get what is a availability zone 🤔

For all intents and purposes you can think of an Availability Zone as a datacentre. Therefore, provisioning a VPC with subnets in multiple AZs gives you redundancy should AWS lose service in one of their datacentres/AZs. I’m not familiar with Fargate but having subnets in multiple AZs doesn’t have a cost implication, however if you need to have NAT Gateways for those AZs, they do incur a cost.