An operand expression can be: a literal, any element available in the scope, or a sub-expression.
A constant that you want to use in your expression. Literals can be strings, numbers, boolean values, etc.
Any element writable or not that is available in the current scope of your expression. These elements can be parameters of any kind, local variables, session variables, site properties, function calls and runtime properties.
Service Studio provides some built-in functions. A built-in function always returns a value and can take zero or more parameters, separated by commas. See Built-in Functions.
You can also call a User Function. A User Function also returns a value and can take zero or more parameters, separated by commas. See how to use User Functions.
In Service Studio there are some elements that provide runtime properties, such as widgets and timers. These are properties that are instantiated at runtime. See About Runtime Properties.
In fact, besides literals (which must be typed), an expression operand is basically any element that can be selected in the Expression editor's Scope Tree.
Expression Editor | Scope Tree | Expression Operators | Timer Runtime Properties