This is the second post in a series of posts treating software quality attributes. Unless your only concern is the performance of your software I do recommend that you start by reading the first post of this series.
The performance of a software is quite simply how fast the software does it’s job. It is commonly measured in response time, that is how long time passes between the initiation of a command until the command has been completed. Performance is often measured in seconds or milliseconds in combination with workload, that is how many concurrent requests the system is handeling. That is the simple part of thinking about performance.