When a message is relayed throughout your body, a series of chemical transactions take place between nerve cells relaying information over specialized junctions called synapses. This process can be seen as a conversation between neurons.
In the realm of neurons, there is post-synaptic potential (the potential stimulation a neuron can transmit) and action energy (the amount of energy required to perform a function such as flexing a muscle or blinking your eye). A neuron can produce a post-synaptic potential of 1 mV (millivolt); however, an action might require much more. Summation is the process of getting an aggregate of all the required post-synaptic potential and then transmitting.
One type of summation is temporal summation. Temporal summation is the process of combining the post-synaptic potentials of several neurons that are fired sequentially. If fired in rapid succession, these potentials (e.g. 1 mV+1 mV+1 mV) will become one cohesive sum (e.g., 3 mV). Temporal summation is summation that creates aggregates of action potentials fired at different times.
Spatial summation creates aggregates over space rather than time. If action potentials are produced simultaneously, through spatial summation, these potentials can be combined to create an aggregate. This is possible due to the space constant in cells, which allows a charge in one area of the cell to spread to other regions of the same cell.