How to test strategies and back test them

Hi,I am new to day trading and i see everybody talking about testing strategies and backtesting them,saying i have 65% success rate something or other.Like how do i do it and what is meaning of strategies here like you can’t predict the market right,a bot can’t do that right then how do they do it and what is the actual meaning behind it.please explain in detail about it and how to do it.