I am a non-english speaker and I am finding it really hard to understand. Could you break it into simple parts?

On any turn, the player can choose a grid ,which has a positive integer in it, from the sub matrix, and decrease it by a positive integer, which less than or equal this grid's number. Don't know if some comma helps.

you can choose a grid （x，y） from the sub matrix （the X1，Y1 to X2，Y2） decrease K （K>=1 AND K<= the number of （x，y））

The commas helped. Thank you both of you :)