A plus sign is the sign + that is put between two numbers in order to show that the second number is being added to the first. It can also be put before a number to show that the number is greater than zero (+3), and after a number to indicate a number that is more than a minimum number or amount (18+). More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.