The squiggly lines that appear on some plant leaves are usually the result of leaf miners, which is a general term for the larvae of multiple different kinds of insects. Leaf miners live inside plant leaves and feed themselves by burrowing through the leaf tissue, which creates the trademark squiggly lines in the leaf.
Some leaf miners stay localized to one part of the leaf, leaving a spot rather than a trail of destruction. If the leaf miner is still alive and active within the leaf, there may appear to be a dark spot at one end of the squiggly line trail; that dark spot is the leaf miner itself. The least harmful way to control leaf miners in the garden is to simply remove any affected leaves from the plant, as this will prevent adult insects from laying more eggs.