An important element of establishing a claim to another person's land using the legal doctrine of adverse possession is that trespassers must be "open and notorious" in their use the disputed land. That's on top of the other requirements that the use be hostile, actual, exclusive, and continuous for a certain number of years (usually set by state statute).
What does open and notorious mean, though? In practice, this requirement is a legal jargony way of saying, "You can't be sneaky about squatting on or taking over this bit of property."
Consider this adverse possession situation: Bob wants to establish adverse possession and gain title over his neighbor's pond. Every night at 2 a.m. for 30 years, Bob puts on his bathing suit, sneaks into his neighbor's yard, and swims one lap. He then dries off and runs back to his own house.
Now consider whether this matches the elements of adverse possession: Bob can establish that his use of the pond was hostile (without his neighbor's permission), continuous for the statutory period (30 years is far more than most states' laws require), and exclusive (assuming no one else took part in his moonlit swims or otherwise used the pond).
But Bob's claim runs into problems when he gets to the requirement that his possession was "open and notorious." He was using the property only in the middle of the night, quickly, when no one would reasonably be expected to see. Having acted in a sneaky fashion, Bob has failed to put the true owner of the property on notice.
You might wonder, why would a court care whether a trespasser is surreptitious in his trespassing? If he meets all of the elements of adverse possession; which already rewards trespassing, despite its moral questionability; what difference should the manner in which the person used the land make?
The policy rationale behind requiring open and notorious possession is that the true title owner of the property should be given every opportunity to notice the trespasser's behavior and put a stop to it. If a landowner sees a squatter on his lawn, and doesn't care, then perhaps it makes sense that after decades of use, the squatter should establish that claim of title.
But if the squatter isn't openly visible, and in fact hides signs of presence on the land, how could the true owner possibly know to retain a lawyer or call the police? Courts do not reward these sorts of sneaky trespassers with title through adverse possession.
Many courts interpret the "open and notorious" requirement to mean that the trespasser must act in a manner consistent with ownership. In other words, if a trespasser is trying to claim title to your backyard, he can't appear on the land, hiding behind trees, and only in the dead of night.
Trespassers would need to treat the yard the same way that they'd treat their own yard: for example, by barbecuing there in the afternoon, inviting family over, and installing lawn furniture. These sorts of visible acts of ownership over the property would put the true owner on notice that another person is now using the land. This gives the owner the opportunity to hire a lawyer or call the police, for example. Or, the owner could simply grant written permission to use or rent the land, making the nature of the relationship clear, while preserving one's title and interest in it.
In other words, a trespasser must be able to show that he or she used the land such that any reasonable person would have thought the trespasser owned it, and that the owner nevertheless didn't act to stop this.