How do insects help in pollination?

Hint: The term pollination refers to the process of transfer of pollen grains from another of stamen to the receptive stigma of the carpel of the same or genetically different flowers of the same species. It is of two types: a) Self-pollination, b) sCross-pollination. Pollination occurs by means of Abiotic and Biotic agents.

Complete answer:
Insects pollinate flowers when they go in search of food. The flower produces a sugary liquid called nectar. When they reach the flower and by landing upon them, pollen grain sticks to their bodies, when these insects travel from one to another flower these pollen grains transfer to the stigma of flowers. Pollination by insects is called entomophily. Bees are the oldest group of insects and alone pollinate about $80\%$ of entomophilous flowers. Entomophily is the most common and highly specialized type of pollination. For the continued visit of insects, the flower provides them with some floral rewards: they feed the insect, nectar, and shelter.
The important characteristic feature of the Entomophilous flower is:
Flowers are large in size
Brightly colored
Sweet-scented
Produce nectar
Edible parts
Provide shelter to insect
The stigma is long and sticky enough
Flowers must strong to bear enough the weight of an insect

Note:
In some cases, plants make flowers more attractive. So that the insects easily pollinate the flower e.g. Mussaenda one sepal is enlarged leaf-like and showy called advertising flag.
In some species like yucca, there is an obligatory mutualistic symbiotic relationship between Yucca and its insect pollinator yucca moth {pronuba}.