A tag is a word that you associate with an image to make it easily searchable. When typing in tags, use commas to separate the words to create different tags.
For example, if your image is a man cycling a red bike, you would tag it with:
Man, Red, Bike, Cycling
This means that if someone wanted to search for a picture of a bike, or even a red bike, they could easily search these two tags to get a selection of Blips with red bikes.
To search specifically for a tag add a # before the word you are looking for. If you search without a # at the beginning it will also look for the word in the entry title and details.