Throughput is a measure of a number of work items per a particular time unit. In the context of a network, it is the amount of data that can be transferred from one location to another in a given amount of time. It is typically measured in bits per second (bps), megabits per second (Mbps), or gigabits per second (Gbps).
In the context of application logic, throughput is how much processing can be done in a given amount of time. An example of throughput in this context would be the number of transactions that can be processed per second.