Folks want to know what is happening with their citrus leaves and if it will kill their tree.
This damage is caused by the citrus leafminer (the larval stage of a small moth) and it is mostly cosmetic damage. Mature trees can easily tolerate this damage. Young trees may need to have some horticulture oil sprayed on the new growth to keep this damage to a minimum.
For more information see this publication: http://trec.ifas.ufl.edu/mannion/pdfs/CitrusLeafminer.pdf