In addition to Philipp's list, there's some other contextual episodes worth noting.
TNG S4E03 Brothers. This helps to explain
There are several inside references and jokes in PIC S1E09 and E10 that play off of that episode. You'll also need this to make sense of TNG S6E26 and S7E01 Descent (mentioned in the other answer)
TNG S7E10 Inheritance helps a great ...
Aside from the answer above regarding Picard, the Borg, Seven, and androids, here are some other episodes dealing with the other big part of the show: the Romulans.
The Original Series episode "Balance of Terror" introduces them.
While the Romulans appeared previously in the Star Trek Enterprise, the fourth season episodes "Babel One", and "United" showed ...