What is a sleep association? Quite simply, it's what a baby learns to rely on as a way to fall asleep.
To further explain these, I'll refer to them either as a positive (or baby led) sleep association, or a negative (parent led) association. Neither of these are "good" or "bad", but it can be helpful to identify why your baby may not be linking sleep cycles.
A positive (or baby led) sleep association is considered to be present when a baby initially falls sleep, remains constant throughout their entire nap / night sleep and is the same when they wake up. Examples are turning white noise on and keep it playing throughout the entire nap or night sleep, being popped into a sleeping bag which will keep them warm and cosy, having a cuddle and the same familiar song sung to them before going into their cot, the cot being the place they fall asleep, having a comforter that they can hold & snuggle, sleeping in.a super dark room (regardless of time of day). The predictable familiarity of these associations can help your baby relax and unwind, so they know it's bedtime. At any point your child wakes up, they'll still have the white noise going, they''ll be warm in their sleep sack, it will be nice and dark and they'll have their cuddly to snuggle with - everything is the same as when they fell asleep which is reassuring to them so they can wriggle around, get comfy again and are more likely to drift off back to sleep.
Negative sleep associations (or parent led) are considered to be present when a baby initially falls asleep, but change or stop once they are asleep. This includes anything you are actively doing to assist them in the first place but stop doing it once they’re asleep. Examples of this are feeding (breast or bottle), movement and motion such as the car / pram / rocking or bouncing, holding or patting then transferring to their own sleep space, a dummy which falls out and they are reliant on a parent to replace it, or music / white noise which stops playing after a certain time.
Despite the name, there is nothing wrong with parent led associations, and if it's working for your family, there's no need to change anything!
However, if your baby is waking frequently overnight and / or catnapping during the day, you may find this helpful to pinpoint why they are struggling to link sleep cycles.