"Pre-Clinical" means evidence from lab studies on how the treatment or substance rather than studies on people with cancer (clinical studies). These studies will usually be found in scientific and medical journals. Occasionally, proponents of alternative therapies will cite unpublished pre-clinical data. If the data is not published obviously it is extremely difficult to evaluate.
Pre-clinical studies include studies on cancer in animals, most often mice (referred to as In Vivo studies) or on cancer cells grown in "test-tubes". Cell culture studies are called In Vitro (meaning "in glass"). Pre-Clinical studies are a much weaker form of evidence than clinical studies. Most things that work well in mice and in test tubes do not work well with humans who have cancer. This is because mice have different metabolisms than humans and drugs can affect them differently, and because the cancers tested in mice are "model" systems which, while intended to be more or less similar to human cancers, may be vastly different. In cell culture studies, the cancer cells may be cells maintained for a long time in test tubes and they may have acquired different characteristics than natural cancer cells, but more importantly a test tube is a vastly different environment from a human body. Drugs in a human body are transported and metabolized entirely differently, and the tumor itself is a much more complex entity than just cells growing in culture. It has blood vessels, immune cells, and more.
Although pre-clinical studies are much weaker than clinical studies they can provide background support which at least increases the plausibility of an alternative treatment. While they are weak, pre-clinical studies by themselves might justify taking a substance which is proven to be non-toxic and which is not likely to interfere with other treatments. You should however be extremely wary of exaggerated claims based only on pre-clinical studies. This is particularly common with those hawking supplements, especially those sold through Multi-Level Marketing Organizations.
It is more encouraging if there are many different studies from different scientists and different institutions rather than just one study which has not been confirmed or replicated.