This is in regards to creatures that have the multiattack ability and PCs, like monks, who have flurry of blows and multiple strikes per attack. Each time the creature attempts to make a new strike in their multiattack, are they required to make a new attack roll against the opposing creature's AC or is the original roll good enough for all following attacks? Does this apply to bonus action attacks like flurry of blows or off-hand weapons? Does the original roll count towards a second target of a multiattack, should the attacker switch targets?
...the rule is simple: if you're making an attack roll, you're making an attack. (PHB p.194)
Since you are making multiple attacks you make an attack roll for each one. This is true even though you may not be taking the Attack action multiple times. This applies whether the source is bonus action (Flurry of Blows, two-weapon fighting), Extra attack (multiple attacks as part of the Attack action), or some other source (Haste, for example, granting an additional action).
Note that I'm equating "unarmed strike" with "an attack"; I get that reading from the "unarmed strike" entry in the weapon table on p.149 (of the first printing) or from reading Melee Attacks on p.195 (subsequent printings).