Consider these two switches. The one on the left is connected to the one on the right with an uplink rated at 100Mbps. It has 8 other ports, also rated at 100Mbps each. All data destined to a device on the right hand switch has to be transmitted over the uplink connection which is connected to port n.
Port n can only operate at 100 Mbps so this is the ultimate limit on throughput. If the other 8 ports all generated traffic at the maximum of 100Mbps then this is equivalent to a network traffic load of 800 Mbps. This cannot be transmitted over a 100Mbps port without incurring delay and packet loss. Therefore when cascading switches, think about the capacity of the uplinks and the potential traffic load being placed upon them. In the above case, it would be advisable to consider an uplink port speed of say, 1Gbps; keeping the other ports at 100 Mbps.
This is problem is often referred to as the funnel issue.