Normally what they want is an official letter from your bank with the letter head showing your current bank balance, account holder name, last 6 months average balance, etc... (you may check IRCC website for more details). If your last 6 months average balance is much lower than the required amount, that's when you need to show where this sudden money come from. There are variety of reasons why sudden influx can be there: Some move money from one account (current) to the savings account to meet the minimum requirement, some get money from their parents/relatives/friends (not borrowed), some sale of possessions (cars, houses, jewellery, etc...). The important thing, it is ok to suddenly increase your account balance to meet the minimum requirement (although there is general feeling of you can't create EE profile without first having sufficient balance in the account as that might be seen mis-intrepertations.); However, you must show proof of where this came from. Gift deeds, Sale deed etc... Along with LOE.
You may search in this forum. There is a big thread focusing on Proof of Funds so you will find many cases and their answers from members who already got PR.