Dispatchable power generators are those that can be dispatched (i.e. instructed to generate power) as and when required by the power system operator. In other words they are controllable, able to raise or lower their output as and when needed to balance supply with demand.
In practice, some sources are more “dispatchable” than others, in terms of how quickly they are able to react to changing demands. So, for example, nuclear power plants typically require substantial time to ramp their output up or down by significant amounts, more-so than coal and, in turn, combined cycle gas. Open cycle gas and diesel generators can react rapidly to meet short-term peaks in demand.
In the absence of storage, renewable power sources which depend on the weather – wind and solar – are inherently non-dispatchable (beyond an ability to turn output down if instructed). On the other hand others like hydro and biogas power can be highly dispatchable.