What is the word/phrase which describes that type of shade which one can witness on a sunny day under a tree which doesn't have very thick foliage; the one which is a mixture of shadow spots formed by the leaves and light spots from the sunlight coming between them?
What about dappled shade?
Marked with spots or rounded patches.
‘Nam's yard sat soft-lit under a few swinging lanterns amid dappled shade from the trees.’ (ODO)
The best place to be today was in the woods, in the dappled shade of Barming Woods, away from the 27.C heat! (Source)
I really like the answer given by Max, but if you are looking for a less poetic expression with a greater likelihood of comprehension (I feel there might be a few people who might not understand "dappled shade", as good an expression as it is), I would say "patchy shade". On the other hand, if you want a more poetic expression, Max's answer is better.