A bot is an unreal profile designed to act like a real member and to interact with the other users. This feature mostly can be used at the very early days of a community, when initial contents are going to be added. Each bot has a face, name, and personality, which makes it real and trustable.
Community admins/moderators can use the face of bot accounts for posting content, replying to a discussion or an article, answering to a question, and up-voting others' answers. The purpose is to build an engaging environment and to encourage all the members to interact with each other.
Once a bot is added to your community, admins and moderators can use the face of the bot to do normal activities in the community.
For example, when you want to answer a question by a bot, click on the "Answer" button like a regular member. When the answer window opens, you see a small arrow right next to your profile picture. Click on the arrow to see the list of all your bots. Click on one of the bots and start to type down your answer, and post it. It will be posted on behalf of a bot user, not the admin's account.
Do the same when you want to ask or answer a question, post article or discussions or a post, up-vote an answer, reply to a discussion or an article, etc.