Hostels are quite basic forms of accommodation where guests stay in private or dorm rooms. Hostels are most often known for their dorms, which will have multiple beds in them to allow more people to sleep in the room. This is the cheapest type of accommodation you will find when travelling (except possibly camping!)
Many hostels are generally of a very good standard. They are clean, have good facilities and may even provide you with breakfast. However, some hostels are not as pleasant. Luckily, there are websites where you can look hostels up before you book and see what other travellers like you have been saying. This is a good way to get honest information and to plan your trip accordingly.
Staying in a hostel can be a great social experience. People tend to be friendly and there are lots of solo travellers who want to meet new people. Find a hostel with a common room and go and make new friends.