This is the amount of money in an account. This could be a credit balance (money you own) or a debit balance (money you owe, for example an overdraft on a current account).
AER stands for the Annual Equivalent Rate and shows what the interest rate would be if interest was paid and added to the capital balance each year. The higher the AER, the better the return you will receive.
Assets are things that a person or company owns, such as a house or money.
ATM stands for Automated Teller Machine. They are known as cash machines and can be found in many public places. They allow you to check your balance and take out money from your current account by using your cash or debit card and entering your PIN (Personal Identification Number).