Problem: there is a big gap between the side of the door where the lock is and the frame of the recess, so that the lock latch doesn't reach the strike plate to keep the door shut. It takes a slight pull to open it.
What can I do to fix it?
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This looks to be an interior door. The best way to fix this is to take off the door casing trim at the latch side on both sides of the door opening. Then using opposing pairs of tapered shims sticking in between the door frame and the adjacent stud you pound in the shims so that the lower portion of the door frame is crowded over closer to the door. I would use at least three or four sets of shims to move the frame out in a uniform manner.
Before starting this make sure the striker plate screws are not long ones that go all the way into the stud. If so remove them before using the shims.
After the shims have moved the frame over then you may need to add additional finish nails through the face of the door jamb through the shims and into the stud to hold things in the new position. After that you can work at replacing the casing trim.
I notice from your picture that the bottom hinge side of the door looks like it has no clearance. This may be due to the fact that the hinges were inset too far into the door and / or frame. If this is the case it is a good bet that this is part of the reason for the excessive gap on the door latch side. You may want to look into this and see if shimming out the lower hinge (and possibly also the center one) to see if this would fix the gap problem on the other side of the door. If so that would make a simpler solution than removing the casing and shimming the frame on the latch side.