I don't understand it.
Well a wave is just a description of dynamics... it can apply to the position of an element of a string that's vibrating, or to the water level in a tank, or to pressure values in the atmosphere, or electromagnetic waves.
The wave is just a description of how perturbations (change) travels spatially through such a system as time passes.
We often take "wavefront" to mean the line along which the quantity that is "waving" is at a maximum. For example, with a water wave, it would be the line formed by the top of a ripple.
We could also use the minimum point (bottom of a ripple) or halfway in between, provided that we're consistent about it.
For a wave in three dimensions - as with a sound wave from a loudspeaker or the wave from a radio antenna, the wave front will end up (after the wave has settled down, a few wavelengths from the source) as a spherical shell of points that are all at the maximum, minimum or any other particular phase of the 'disturbance'.
The concept of a wave front is often useful because it represents a set of points that are equidistant from a point of origin (or some image of that point) or perhaps from a point where the wave will be focussed and all parts of the wave will arrive at the same time. However, this only applies when the wave front is the right shape and sometimes. points on the wave front will not arrive in step and the image will not form correctly (as with a badly focussed image on a camera sensor)
I suggest you Google Wave Front Images. There are many pictures of wave fronts used to explain a range of different phenomena. These may not give you a 'definition' but will show you what it's all about.
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