
Soundcheck

A soundcheck is the preparation that takes place before a concert, speech, or similar performance to adjust the sound on the venue's sound reinforcement system or PA system. The performer and the sound crew run through a small portion of the upcoming show to make sure that the venue's front of house and stage monitor sound systems are producing clear sound, that they are set at the proper volume, and that they have the correct mix and tonal balance (a process done using the mixing console). 
Soundchecks are especially important for rock music shows and other performances that rely heavily on sound reinforcement systems.
Processes.
Soundchecks are usually conducted prior to audience entry to the venue. The soundcheck may start with the rhythm section, and then go on to the melody section and vocalists. After technical adjustments have been completed by the sound crew, the performers leave the stage and the audience is admitted. Since the acoustics of a venue often change somewhat once it is filled with audience members, the sound engineer often has to make minor modifications to the sound system settings and levels once the audience is there. 
If there is more than one artist performing, soundchecks can be more complicated.
Recording and rehearsal.
Some acts will do multi-track audio recordings of one or more soundchecks. Those recordings will be used as the sound sources for future soundchecks, to reduce or possibly eliminate the need for the performers to be present. Such 'virtual soundchecks' are also sometimes used to tune the PA system to accommodate the acoustics of a new venue. 
Artists sometimes perform light-hearted or off-beat numbers during soundchecks; some such as Paul McCartney later include a few of these on their live albums.
Artists who change their set list during the course of a concert tour often use soundchecks as a rehearsal opportunity to try out new material or revive old material, to see if they want to introduce such material into the concert proper.