Upvote in social media terms simply means that the post is being approved by the person who upvotes it. When people thinks that a particular post contributes to either a trending topic, conversation or a particular question, they tend to upvote such post or comment to show that they approve of what the person posted or commented.
In cases whereby they think the post or comment does not contribute to the question or topic or is probably off-topic, a "downvote" is used.
The higher the number of upvotes someone has on a post, it shows that people are really in approval of what was posted.
