Kolu is a popular (South?) Indian custom / force of habit celebration during the religious festival of Navratri. Nav (not him) means 9 and ratri is night. Navratri therefore means 9 nights. A celebration of the three goddesses of Hindu mythology.
In the Kolu (also spelt golu) families arrange dolls (used to be clay dolls. Now plastic and metal make their appearance too) on steps (steps are always odd numbered) and invite other families to come visit. There's a bit of competition in these visits. A gentle one-up-man ship, as people see who has the better display.
One reason people have the golu, apparently, is to promote local artisans and local crafts. Dolls used to differ from region to region - wooden dolls had different features while clay ones would be different - and this festival is a great way to bring them all in one place.