As usual YS has it nailed. I would add:
The brake has worn to the point where there's too much free play. The RH brake pad is slipping badly off the braking surface of the rim. Possibly the frame is distorting slightly but that's because you are squeezing that lever far harder than you would need to to effect a good stop if the brakes were set up correctly.
The cable looks to be moving freely so this is what you need to do:
Tighten the cable by using the adjuster on the brake lever, screwing it outwards to effectively "shorten" the outer cable. Get the right sized hex key for the long nut on the brake pad. Loosen the nut and adjust the pad so that by pulling the brake lever it can just hold itself it in the correct place against the rim. This is a tricky adjustment to get right and it might take a couple of attempts, squeezing the brake lever while moving the pad around. Make sure the pad isn't rubbing the tyre or likely to come off the braking surface as it wears; it needs to be well squared up to the rim. Nip up the long nut very gently and re-check the movement. Then, holding the brake pad with your fingers to prevent it from rotating, tighten the long nut fully.
Do the same for the other side. Adjust the effective length of the cable outer so that, as YS writes, there is minimal clearance between pad and rim. Once you've got this right, lock the cable adjuster by tightening down the larger ring on the adjuster so as to prevent the adjuster from screwing inwards.
If one brake arm seems to be reluctant to move and the other moves too much you can adjust the spring tension, increasing it by doing up that small bolt you can see under the pivot. Go carefully because it's a tricky adjustment.
If your rear brake has worn a lot (the bike looks quite new) it's because you are using it too much in preference to the front brake, which is always cleaner and drier and should be doing 80% of the braking in dry conditions. In the wet it should be 50/50 front/rear.
