Most people, when asked this question, will normally think about what weight they can lift, curl or press in a gym using barbells, dumbbells, or on a weights machine.
You may be bench pressing huge weights and curling the biggest dumbbells on the rack, but my guess is you are not that strong, and this may come as a bit of a shock to most people.
True strength comes in the form of functional fitness, fitness that can be applied and used in everyday situations, not just in a gym.
When you carry the shopping from your car boot to your kitchen, or carry your child to bed, you use many muscle groups which work together to achieve the job in hand. The body works as one unit, integrating all the muscles together.

Weight training in its conventional form, isolates muscle groups, and it does not teach the muscles it isolates, to work with others.
Functional exercises focus on building a body capable of performing real life movements in real life positions, whereas weight training on machines in a gym, teaches the body to perform only in positions when posture is made ideal by the machine.
If you have ever reached for something or bent over to pick something up, and we are only talking about small, light items such as a bottle of water, and felt your back go, then your functional fitness is seriously lacking.
This can be attributed to poor core strength. When we perform everyday movements, such as carrying the shopping, our body calls into play many more muscles than would be used on a gym machine. When we lean over or reach, to lift the bags, we use the stabilising muscles, which include the core muscles of the abdomen and secondary muscles to keep our balance so we don’t fall over.

When we use gym machines, our stabilising muscles are not used because the machine has taken the need to keep balance away. If we are not careful, the stronger muscles get stronger and the weaker muscles get weaker which causes an imbalance in the way our bodies work and function.
This explains why a seasoned user of weight training equipment who is capable of pressing a weight far above that of their own body weight over their head on a shoulder press machine, cannot perform a single repetition of a push up in the handstand position.
The machine has taken away the need to balance and that person is only strong when using a machine.
The key to functional exercises is integration. It is about training muscles to work together and not independently of each other.
To become truly strong and improve your functional fitness, you need to ditch weights altogether.
It is not surprising to find people who are capable of pressing 500lb on a leg press machine to become a quivering, sweating wreck after only 50 free squats and stop with no weight at all, which takes about 90 seconds! Try now and you will see what I mean.
Stand up straight with your feet shoulder width apart, toes pointed forward and bend your legs and squat down to the floor, keeping your back straight. When your bottom touches your feet, press back up again and repeat this over and over at the rate of one repetition every 2 seconds.

When performing this movement, you use far more muscles than when lying or sitting on a machine, which is why it is so much harder. In fact, all body weight training is much more demanding than weights machines and will shock most people.
If you are looking to improve your muscular strength and endurance, then body weight exercises far out-perform weight training methods. Just take a look at wild animals, they do not use weights, gym machines or isolate muscles in any way when they go about their daily routines. It surely is no surprise that going to a gym and using weight machines and following split routines that isolate muscles, doesn’t improve functional strength at all, when you consider a monkey, which isn’t that different from us, swinging about the trees all day with nothing but their own body weight to shift, has the strength of ten human beings.