If you get 2 levels of rogue, you can hide as a bonus action which is helpful during combat because in the full release, hiding during combat will take a full action. You can do three things on your turn in a round of combat: an action, a bonus action, and move.
Actions are what you use to attack and cast (most) spells. A bonus action is a simple thing like drinking a potion, attacking with an offhand weapon, jumping, etc. So you can imagine that being able to hide and also attack on the same turn can be useful to get advantage. Hiding and casting a spell is only useful if the spell offers an attack roll, which bards don't have a lot of. So I would only recommend taking rogue levels if you plan on being a martial-bard (valor or swords). If you just want to be sneaky, bards can do that fine without taking rogue levels.
