It's the percentage of the gold, out of 1000 parts 750 is the amount of gold which makes it 18k. Pure gold is 24k which is too soft to use. So 750 is the gold and 250 are the other metals.
The numbers in 750 gold mean the gold is 18k. The term 750 refers to the amount of gold in the metal. It means the parts per thousand. The remaining metals are other alloys such as silver, copper, or nickel.
"750" means 18K gold. It means that a piece of gold is 75% gold and six parts other metal, or combination of metals. This is a common stamping on gold jewelry in Europe.
The amount of gold in your jewelry is defined with karatage. Jewelry can be up to 24 karats gold. The 750 basically means it is 75% gold, or 18 karats.