Oversubscribing is basically when you go from many ports to fewer ports. So for example, a switch has 48 ports on it but only two uplinks. You can see right away there might be a problem if all 48 ports are being used and the traffic is being shoved down two ports. This brings you to a 2.4:1 ratio assuming the access ports are 1G and the uplinks are 10G which is typical. This is oversubscribing. When you oversubscribe you get congestion. When this occurs, QoS is required to protect important traffic.
Below are the recommendations and good starting point.
20:1 = Access>Distribution
4:1 = Distro>Core
1:1 = Data Center