Generally Grab activities require human involvement to complete their task. Only after the user completes the Grab task associated to the activity, the instance moves to the next flow object in the process flow. If the Grab activity is of type From All/To All then the user must select the next flow object.
The Engine processes Grab activities using interactive threads. If the implementation of the Grab activity is of type screenflow, then the engine releases the interactive thread at the beginning of the screenflow and obtains a new interactive thread when the user completes the screenflow. When possible you should use screenflows to implement Grab activities because they optimize the use of interactive threads.