PLEASE HELP

Hi hoping someone can help me here 
New to this stuff tbh what does "dpo" mean? 
Also I feel as though I have seen and extra extra faint line on a test can't get a photo of it due to that but can see slightly with a light on my phone 
Ocording to this I'm 4 days of my period is that to early to be testing? 
I've had sex on on the revelant days this month when is the best time to test? 

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