It means I strongly agree with you, that I add my one vote, plus one, to what you wrote. On messaging apps such as Slack people use an emoji that shows the plus sign and the digit 1: +1. Instead of writing a new post saying that they wholeheartedly agree, they just put +1 beneath the post they agree with. This is more than putting a Thanks on this site signals wholehearted agreement which was the case for me here because I agreed with everything and loved "I thought I heard it all".

I never thought that I would be explaining emojis to anyone because I am usually slow to learn them, but I guess there is the first time for everything