What do these symbols mean?
The answer is stated in ICAO Annex 12 (Search and Rescue)PDF. (See Appendix I relevant to Chapter 5, 5.8 of the Annex.)

Ground-air visual signal code for use by survivors:



Ground-air visual signal code for use by rescue units:




  Note 1.— Symbols may be formed by any means such as: strips of fabric, parachute material, pieces of wood, stones or such like material; marking the surface by tramping, or staining with oil.