When i turn on my heater it makes a clicking sound. what does that mean

The clicking, if from the dashboard area, could be from a climate control motor. One motor changes temperature desired from hot to cold, and everything in between. Another motor will select "mode" - dash vents, floor vents, defroster vents, and every combination. Have a look to see which one Is affected; you can move the knobs and see which one causes more clicking from your adjustment.

That happened to me and the vent actuator motor had some gears get melted in it. The sound you probably are hearing is the vent door flipping back and forth. Mine got stuck on heating and was putting out well over 200-degree air no matter the temp setting on the passenger side. This is the part that I replaced to fix it in about an hour in my college's parking lot https://www.amazon.com/gp/product/B00OAKGWNA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1