Plasma weapons have 2 fire modes: auto-fire and charge fire.
For both modes, you will have a overheat gauge. The gauge will go up while firing in auto-fire and while charging in charge mode.
The principle will be that the more the gauge will go up, the more your attacks will be powerful, but you take also the risk to totally fill the gauge (overheat) and then get staggered and take small damage.
Plasma weapons have heat venting; you can lower the overheat gauge with the [R] reload key.