There could be several possibilities depending on what your doctor meant exactly.
One possibility is that the doctor meant that he/she cannot fully correct your myopia with LASIK, and that you would expect to have residual myopia even straight after any attempted LASIK correction.
In this case, you should know that
1. LASIK reduces myopia by changing the shape of the cornea, and it does this by flattening the centre of the cornea.
2. This will cause some corneal thinning-the greater the myopia to be corrected, the thinner the cornea will get.
3. In order to avoid complications like corneal ectasia, surgeons will make sure the cornea does not get below a certain thickness after LASIK.
4. If by observing this limitation, your myopia cannot be corrected fully, then you would expect to have residual myopia even straight away after the LASIK. Which kind of defeats the purpose of having LASIK in the first place.
For example, let's say your corneal thickness is 550 microns, and your myopia -14D, and we want to maintain at least 250 microns of unlasered cornea to avoid complications. If we 'laser away' -14D of myopia, you might have only 230 microns of untouched cornea, which is dangerous. Perhaps if we lasered away -12D of myopia, you might have around 250 microns of untouched cornea, so -12D of myopia might be considered the maximum myopia you can be treated for. In that case, you would be left with -14 - (-12)=-2D of myopia straight away after the LASIK. Note that nowadays many doctors try to leave more corneal thickness behind than 250 microns. Also please note that the above numbers are only for illustrative purposes and are not meant to apply to any particular person.
The other possibility may be that your doctor means you have a higher risk of regression-ie there is a higher chance of myopia returning in the future even if the original LASIK treatment gives a good outcome. I have written an answer to why the risk of regression is higher in high myopic corrections elsewhere.
