Strength refers to your ability to apply force. When enough force is applied an object will move. Whether you're trying to move your own body or an external object.
We typically measure strength by the amount of weight you can lift. What is the heavies deadlift you can perform? How much weight can we strap on your back and you still complete a pull-up? This can be measured in a one rep max effort or in multiple reps. For example if we put 200 lb on a barbell how many times can you complete a squat before you have to set the bar down?
While strength is just one of the ten general physical skills we train. There are some people that beleive that strength is the be all end all to fitness. They would be mistaken. Depending on the person strength may be a weakness and you may want to bias training in that direction until your strength catches up with the rest of your fitness but that doesn't mean you should ignore the other aspects of fitness.