IP Ratings measure the ability of lights to prevent dust and moisture entering the light body. 

IP Ratings are made from two digits ie. IP45 where "4" is dust or particle and "5" is moisture. 

The first digit represents dust or particle intrusion. 

1. No special protection
2. Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter.
3. Protection against fingers or other object not greater than 80mm in length and 12mm in diameter.
4. Protection from entry by tools, wires etc, with a diameter of 2.5 mm or more.
5. Protection against solid bodies larger than 1mm (eg fine tools/small etc).
6. Protected against dust that may harm equipment.
7. Totally dust tight.

The second digit represents moisture intrusion.

1. No protection.
2. Protection against condensation
3. Protection against water droplets deflected up to 15° from vertical
4. Protected against spray up to 60° from vertical.
5. Protected against water spray from all directions.
6. Protection against low pressure water jets (all directions)
7. Protection against strong water jets and waves.
8. Protected against temporary immersion.
9. Protected against prolonged effects of immersion under pressure.

So IP45 would be 

IP4. Protection from entry by tools, wires etc, with a diameter of 2.5 mm or more. IP5. Protected against water spray from all directions.