Tempo = The speed at which you perform an exercise - sometimes referred to as 'cadence'

Tempo is usually expressed as 4 numbers - For example, “4 0 1 0”, or "20X0"

The First Number: The Eccentric Lifting Speed: The eccentric portion of a lift relates to the phase when the working muscle(s) are lengthening under tension. For example, the quadriceps (thigh muscles) on the downward phase of a squat, leg extension, leg press etc. The first number is usually the biggest number, as the eccentric portion of the lift is usually controlled.

The Second Number: Pause in the Stretched Position: If the number is a zero then this would mean that there is no pause after the eccentric phase of the lift and you go straight into the concentric phase of the lift. A '1' would denote a 1 second pause in the stretched position.

The Third Number: The Concentric Lifting Speed: The concentric portion of the lift relates to the phase when the working muscle(s) are shortening under tension, For example, the quadriceps on the upward phase of a squat, leg extensions, leg press etc. An ‘X’ during the concentric phase relates to the lift being performed ‘explosively’.

The Fourth Number: Pause in the Shortened Position: If the number is a zero then this would mean that there is no pause in the shortened position and that you would go straight into the eccentric portion of the lift.

So a tempo with the numbers '3111' would mean a 3 second eccentric phase, a 1 second pause in the stretched position, a 1 second concentric portion, and a 1 second pause in the shortened position.

What are the benefits to using tempo in your training?

Slower lifting tempos can be extremely useful for beginners who need to learn motor control, create mind-muscle connection and learn correct movement patterns
Can be useful for experienced lifters as means of overloading a movement
Using tempo standardises a lift so you can assess whether you are making strength improvements or simply just moving the weight faster each week.
Slowing down the exercises means less load can be used, reducing the likelihood of injury occurring
Using pauses in either the stretched or shortened position (or both) can vastly improve the effectiveness of certain exercises
Using specific tempos can ensure you are utilising the correct energy system you are wishing to target

What are some of the disadvantages of slowing down the lifting speed?

Slower lifts create less mechanical tension on the muscle
Using faster tempos can result in greater overall work - “Lifting with short eccentric phases and no bottom rest produced greater repetitions and Power Output, and therefore, total work volume than repetition tempos with 4-second eccentric phases or 3-second bottom rest intervals” (King, et al, 2011)
You can use a greater load on the bar with faster tempos
You can utilise the stretch-shortening cycle by using a faster lifting speed
You can develop more strength and power utilising a faster lifting speed
Greater recruitment of fast twitch muscle fibres when lifting faster

So as a general rule of thumb, the newer to training a client is, the more I will utilise slower lifting tempos. This is to ensure that: -

1) Chance of injury is minimised
2) The emphasis is on controlling the movement and learning correct movement patterns
3) To establish a mind-muscle connection - this is much easier with slower lifting speeds
4) To learn how to recruit certain muscles. For example, it might be advantageous to pause at the top of a hip bridge to maximally contract the glutes and 'feel' them working

Only once you have mastered an exercise with a slow tempo should you try to lift faster.

Whether lifting with a slow tempo or a fast tempo the goal should always be to lift with intent & completely focus on what you are doing.