The goal of chainline is for a triple to get the chain straight on the middle chainring and the middle sprocket. The desired result is so that the chain deflection is the same for the little-little position and the big-big position.

You set it by in the front for a triple by measuring from the center of the middle chainring to the middle of the down tube, and that should be 45.5 mm (maybe it's 46.5? I'm sure someone will chime in with the correct numbers) within a millimeter. If it isn't right, you need to check that the BB is correctly installed, and if it is, you need a BB of different spindle length. Actually it's the length on the drive side that is really important. In the rear the freewheel or cassette position is essentially automatically correct if the frame is aligned, the hub spacing is correct as new, and the freewheel or cassette are the type originally intended for the bike. Mismatches with a freewheel bike can be resolved with careful adjustment of the lateral position of the hub between the dropouts, re-dishing of the wheel, and possible choosing a different freewheel. Freewheels can also be shimmed on the hub to move them farther from the center of the bike.

Yours looks like you have a 5-speed rear which normally wants a 120 mm frame. But the big gap between your small cog and the drive-side dropout suggests you actually have a 126 mm frame and hub, which would match up better with a 6 speed or 7-speed.