The ability of devices to connect and communicate with each other or the internet. Like having good cell phone reception - you need connectivity to communicate.
Good connectivity means your laptop can access websites, your phone can make calls, and your smart TV can stream videos.
Connectivity is a general networking concept, not a single cloud service. All major clouds provide many services (VPC/VNet/VCN, VPN, Direct Connect/ExpressRoute/Interconnect, load balancers, DNS) that help you create and manage connectivity between users, devices, and cloud resources.