Proxy. This word has been fascinating for me for a long time. I think I understood more or less the meaning from the first time, but it was quite inaccurate. In this moment I feel much more comfortable to read and use it. A proxy is an indirect measurement of some magnitude. For example in cooking, if you are not able to measure the cooking state or temperature of the turkey you can guide yourself by the time. We use proxies all the time and rely on them very confidently, because they are based on the knowledge of the issue we have.
Proxy measurements are also a source of criticism by skeptics as it is very easy cast a doubt about them. Even more, in some cases they will be right as a particular proxy may be a wrong measurement technique (if you cook pasta in Bolivia, the time measurement is not right, becuase the pressure is low and the boiling temperature changes, OK we have learnt something new about this proxy, its application scope). This is science, learning from errors, getting more reliable data and questioning them and the previous theories.
But even if they are not perfect, proxies are many times the only source of information for many sciences (nobody measured the weight of dinosaurs, or has been close to our sun) and they are one of the fascinating things about science: how indirect measurements, hints in some cases, combine with solid base of science and theories to be proben true or false to get a coherent picture.
Because climate change is not based in a weak proxy, it is based on knowledge of greenhouse effect, many current measurements and many different proxies from the past. And the models are only considered reasonable when they fit those data.