Access / aggregation / core
Servers plug into "access" switches, access switches feed a smaller "aggregation" layer, and aggregation feeds the "core." It is cheaper to start building this way because the upper layers are small.
Every layer is a funnel. Two servers on different access switches have to climb up to aggregation (or all the way to the core) and back down to talk — and each funnel narrows the pipe. It works fine until your traffic goes sideways instead of straight out to the internet.
Detailed explanation
North-south optimized, oversubscribed at every layer, east-west traffic hairpins through higher tiers. Common in enterprise builds; modern hyperscale abandons it for flat spine-leaf because east-west (server-to-server) is now the dominant traffic pattern.