Hi, can you help me to decipher this lines from user manual of Gigabyte GA-P31-DS3L?

"The main power connector is compatible with power supplies with 2x10 power
connectors. When using a 2x12 power supply, remove the protective cover from the
main power connector on the motherboard. Do not insert the power supply cable into pins
under the protective cover when using a 2x10 power supply."

What does that exactly mean? That it is impossible to use 20pin+4pin cables? Or is it just instruction to place 20pin cable on correct side of 24pin connector?

It means the board has a 24-pin input with 4 pins blocked off. If you use a 20-pin PSU, plug it into the 20 pins that are not blocked. In the event that you remove the cover that blocks those 4 pins, be sure you do not plug the 20-pin on the wrong side.

You will be fine using a 20+4 pin PSU.

Thank you, I was confused, because I think that 20pin cannot be inserted into 24pin incorrectly unless hammer is used

The question has already been sufficiently answered, but I am currently running the same motherboard using a 20pin connector.Even when using a 20pin connector both plugs are designed to only fit together the correct way round.You cannot connect then incorrectly unless you use a lot of force.