If a pesticide or fertilizer says that you need to do a foliar application, you are being advised to put it directly on the foliage. Leaves are able to absorb some nutrients and chemicals through the pores on their surface.
During the summer, you should be cautious when doing a foliar application. The label will tell you what temperatures to avoid lest your leaves become scorched. It can still happen, though, even if you aim for cooler temperatures.
For example, I treated some of my fruit trees for an infestation of spider mitesthis summer. I used horticultural oil, which is an organic acaricide.. I tried doing it in the evening when it was not as hot, but some of the leaves still burned. It is always a good idea to aim to use chemicals as little as possible unless necessary. Look for biological and mechanical controls when available.
You should also make sure that the leaves do not stay wet for long periods of time as this can sometime encourage pests and diseases like fungi.