The + (add) operator takes two numbers, variables, or expressions and adds their values together, thus returning a single new value. The + operator appears lower in the order of operations than both * and /, so if those appear in an expression, they will be executed first. In addition, the - operator has the same order of operations as +, so the calculator simply executes them left to right in the order that they appear.
:1+1 2 :5→X :2+3X 17 :2→A:3→B :A/B+B/A 2.166666667
The + operator is overloaded (meaning it has more than one function) by the calculator, and it can be used to put strings together. The strings can consist of whatever combination of text and characters that you want, but it unfortunately does not allow you to join a string to a number (i.e., "Hello5" cannot be made with "Hello"+5).
:"HELLO"+"WORLD "HELLOWORLD :"TI"+"-"+"BASIC "TI-BASIC