This piece of equipment can make your darkroom experience a lot easier since an easel will hold your photo paper flat and in the right position, it can give your photo margins and enable precise measurement of your photo size.
A good enlarging easel should have a spring or mechanism that lets you lift the blades and leave them up while placing your paper in position. Make sure your easel is giving you rectangular prints. If not, it needs adjusting and with some models you have to bend the metal to do this.
The easel should also have flat edges on all four sides which lie on the paper. If you have angled or beveled edges they can reflect light from outside the image area back to the paper. This can cause a slight darkening on your print. Good easels are also quite heavy and sturdy so they don't move when you are putting the paper in place.
The blades of the easel should also come down in the exact same size after you have placed the paper in as when you were adjusting it.
Easels come in various sizes and styles such as adjustable, fixed size and borderless. They generally range in price from about $30 to over $500.