IP transit is the service by which a carrier sells you access to the whole Internet: it carries your traffic to all destinations, over its own network and interconnections. It's billed by bandwidth (Mbps/Gbps), often at the 95th percentile.
In practice
Transit contrasts with peering (free, direct exchange between two networks). In practice, resilient hosting combines several transit providers (BGP multi-homing) and peering via an IXP to optimize cost and latency.