These are the flowers of an oak tree. Oaks are monoecious; they have male flowers and female flowers on the same tree.
This is quite a complicated picture and will need some explaining. The dangling yellowish green catkins in the bottom part of the picture are the male flowers which produce pollen in enormous quantities. While many insects come to feed on the pollen, they are not the main pollinators; oaks are wind pollinated.
The scaly brown things in the middle of the picture are the remains of the bracts that protected the male flower buds and the leaf buds before they opened. At the top of the picture, growing out of the axil between the stem of the leaf and the twig’s main stem, is a tiny pink flower. This is the female flower.
The male and female flowers ripen at different times in order to prevent self-fertilisation. The female flowers on this tree will have been fertilised by the pollen from another tree before these male flowers ripen and release their own pollen. Each fertilised female flower can result in up to four acorns.