A common resource is rival, but nobody can be kept out
Almost anyone with a boat can fish in the open sea, so the catch is non-excludable. But fishing is rival: a cod landed by one boat is a cod no other boat can land. Goods that are non-excludable and rival are common resources, neither private nor public goods. A village's shared grazing land and groundwater pumped by many farms are others.
Each user keeps the whole gain and shares the loss, so the stock is over-used
Nobody owns the sea or its fish, so no single user has a reason to protect the stock. A skipper who lands an extra tonne of cod keeps all the revenue. The cost, a smaller stock that yields poorer catches next year, is spread across every boat. A fish she leaves behind will probably be caught by a rival, so holding back gains her nothing.
Every skipper reasons the same way, and the stock is fished faster than it can renew. The ecologist Garrett Hardin called this the tragedy of the commons in 1968.