If you recently received a bill from CRA for 2019 and this is confusing to you, it probably means that you have never been asked to pay instalments before. CRA generally likes to get its money as soon as possible. Employees have Tax, EI and CPP taken from their paycheques each payday and CRA gets that money. If this is the first time CRA has asked you to pay taxes for the year that you are currently in, then this is the first time that you have owed them more than $3,000 when you filed your 2018 personal tax return.
CRA has taken that amount (let’s say it was $4,000) and they are asking you to pay $2,000 by September 15th and $2,000 by December 15th. If your tax situation is the same in 2019 as it was in 2018 then you should send them the money they asked for. If the reasons you owed money in 2018 is not going to happen in 2019, then you should be able to ignore the notice.