To be fully understood. Initially you probably understood part of ‘it’, but after thinking longer, you fully understood ‘it’.

"To sink in" basically means to comprehend something that's happening, that would pretty much be the description of the phrase

"to sink in" = to have effects, to make you realise the consequences of something, to think about something that has happened extensively examples: - Grief takes a long time to sink in. - You have done something terrible, so let that sink in. I hope this helped!