A load speed index is usually found on the sidewall of a tyre and is what the manufacturers have tested and rated the maximum loads and speeds that the tyre is capable of. It is very important that you choose the correct load / speed index for your vehicle as per the vehicle manufacturers recommendations. It is usually safe to increase the load / speed index, but again, manufacturer guidelines should be followed.
A load speed index gives the weight and speed a tyre is capable of e.g 116D would indicate that the tyre is capable of carrying 1250kg up to a speed of 40mph. Often in agricultural tyres, the load speed index can alter so that a vehicle travelling at a lower speed may be able to carry more weight. It is always worth checking the tyre manufacturers specifications for further details on this.