To explain something (weak): To describe it; to tell its origin, appearance and/or behaviour; to tell how it is similar to or related to other things; to tell a convincing story about it.
To explain something (strong): To be able to predict, every time and in every situation, whether and how that thing will happen, and always get it right.
Cause and effect: A thing is the cause of another thing if, every time and in every situation that the first thing happens, the second thing also happens.
Natural laws: A natural law is an explanation (of the strong kind). It links a cause to an effect.
An example of a natural law: Water boils at 100°C (at normal atmospheric pressure*). The cause is ‘water is heated to 100°C’ and the effect is ‘the water boils’.
* The reason for the ‘at normal atmospheric pressure’ caveat: if you climb to the top of a high mountain, where the atmospheric pressure is a little less, the boiling temperature of water will be a few degrees lower than usual, which is why climbers on Mount Everest have a hard time making a decent cup of tea. If you went to the bottom of a very deep cavern, where the pressure was a bit highter than at the surface, in theory you’d have to heat your water a bit higher than 100°C to get it to boil. In outer space, where there is no atmosphere and thus no atmospheric pressure, water doesn’t take liquid form at all; depending on the temperature it either freezes to form ice, or evaporates to form vapour.