Shell variables are text substitution done by the shell before the command runs, which is why they behave nothing like variables in a program. The important consequence is that an unquoted variable is split into words, and a name with a space becomes two arguments.
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Watch word splitting bite.
Set a name with a space
Before you run it, say out loud what it will print. Then open the answer.
Quote it
Before you run it, say out loud what it will print. Then open the answer.
Guard an unset variable
Before you run it, say out loud what it will print. Then open the answer.
An unset variable expands to nothing, so a command loses an argument and does something different rather than something wrong. ${var:?message} stops the script with your message, which is the guard worth using by default.
Double quotes allow expansion, single quotes do not. That difference is the whole reason both exist, and mixing them by accident is what turns a message into a command substitution.
name=value with no spaces around the equals. With spaces, the shell reads name as a command and value as its argument, which is a different failure than the one you expected.
"$file" keeps the value as one argument. $file splits on spaces and drops empty values. The quoted form is almost always the one you want, and the cost is two characters.
$1 $2 are the arguments the script was called with, and $0 is its own name. Checking that they are present at the top of a script is the guard that turns a silent wrong run into a clear error.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.