Or they may want you to sign up for a room at 1 p.m. and then come back later in the evening to check in. Some places have early morning check-ins allowed, but then you are locked out from 9 or 10 a.m. until 4 p.m. or so. This is so that they can clean the rooms and restrooms. There may or may not be someone at the desk in the middle of the day to help you if you have just arrived in a new city and are looking for a bed. Again, your guidebook should list the specifics.
Some hostels have curfews at night.
They will lock the doors at a certain hour and even have lights off. But, many city hostels have 24-hour access even if you have to knock on the door. The curfew factor may be important to you if you want to stay out late, but in a city like Venice, Italy, the entire town goes to sleep early so the only reason to stay up late is talking to other travelers in the hostel. In Barcelona on the other hand, the city never sleeps, so you might not come in until 5 a.m., but the hostel also lets you sleep late. Hostels do not normally provide linens, but you can pay a bit extra to rent a clean stack of sheets and pillow case from them.