If you know what delay is, jitter is simply the difference in packet delay. In other words, jitter is measuring time difference in packet inter-arrival time.
It is a specific phenomenon that normally exists in bigger packet switched networks. As a time shift phenomenon it usually does not cause any communication problems. Actually, TCP/IP is responsible for dealing with the jitter impact on communication.
On the other hand, when we speak about Voice traffic and VoIP network environment this can be an issue. When someone is sending VoIP communication at a normal interval, (let’s say one frame every 10 ms), those packets could have stuck somewhere in-between the network and not arrive at expected regular pace to the destined station. It is not usual, but the packets could take different routes or get load-balanced through two similar paths where one of those is congested in that moment.
That’s the whole jitter phenomenon. We can look at it as the anomaly in tempo, with which packet is expected to come and the time he was late to really get there.
In this image above, you can notice that the time it takes for packets to be send is not the same as the time in which he will arrive at the receiver side. One of the packets encounters some delay on his way and it is received little later than it was expected. Jitter buffers are entering the story. They will try to remedy packet delay if required and if possible. VoIP packets in networks have very changeable packet inter-arrival intervals because they are usually smaller than normal data packets, and are therefore more numerous, with bigger chance to get some delay.