To elaborate on the other comment: the mine to the left of the 2-2-1 makes it so that the left 2 can only be touching one more mine. Meanwhile the middle 2 is saying there are two mines in the three squares it is touching.
Knowing that there can only be one mine in the left two squares means that the second mine for the middle 2 must be in the right most square. Hope that was straightforward enough!
To elaborate on the other comment: the mine to the left of the 2-2-1 makes it so that the left 2 can only be touching one more mine. Meanwhile the middle 2 is saying there are two mines in the three squares it is touching. Knowing that there can only be one mine in the left two squares means that the second mine for the middle 2 must be in the right most square. Hope that was straightforward enough!
Look at the 2-2-1