@transactions.push(description: description, amount: amount) What's the purpose behind the colons?

it looks like you are passing in a hash as an argument. When hashes are the only or last argument in a method you can omit the {} around them.

this particular call also uses the popular hash syntax using symbols as the keys, followed by the values. this is quicker and easier to read than the hash rocket syntax

another way to call the method would be:

@transactions.push({ :description => description, :amount => amount })

in the above call I included the curly braces and used hash rocket syntax.

note that when using the symbol key syntax, the colon goes after the name of the symbol (instead of in front of the symbol like normal), mimicking the syntax of languages like javascript, swift, etc.

What would happen to the code if it was written as: @transactions.push(description, amount)?

it probably would not work since that is not a valid hash. each pair in a hash has a key and value.