Sessile organisms are living things who attach to a spot and never leave it. If you are a parent of teenagers, you might be picturing your kid on the couch right now, but that is not entirely accurate. The teen CAN actually move when stimulated.(examples: pizza delivery guy at the door, "supper's ready", "WHO LEFT THE MILK OUT?!")
The oysters and barnacles below are true sessile critters. After a short stint as free-swimming planktonic larvae, they attach to some hard surface and are forever part of it.
This presents some challenges.
Food has to come to you if you are sessile. Most sessile sea creatures are filter-feeders for that reason. You suck, grab, stick to, or slurp whatever tiny nutrients happen to float by.
Shelter is either provided by a shell of some sort, or by simply choosing the right spot to attach to when you were a larva. Low tide with it's extreme temperature changes and lack of water for hours presents another challenge. Again, a good shell or lucky choice will be the ticket to survival.
Sex, becomes another challenge when you are rooted to one spot. You can't really go out on a date. The oysters in the top photo get around this problem by spawning. Everybody just agrees to dump their eggs and sperm into the water all at once. This productivity party is triggered by water temperature and light levels.
The barnacles solve the sex problem in two ways. First, each barnacle is a hermaphrodite, so your neighbor is always a possible mate. Their other solution is to have what amounts to the longest (relative to their size) male reproductive organ in the animal kingdom.
This allows them to "reach out and touch someone" even if they don't live next door.