to approve of an idea or of a person or organization and help them to be successful
to officially support a proposal made by another person in a meeting
They are like and both can mean "be behind someone".
In StackExchange, I often see someone will comment like I second you/your ... to show their agreements to the answer above.
To the case above, can second be replaced by support?
What difference between them?
When should I prefer one over another?