Solid fat is any fat that stays solid at room temperature, as opposed to oils, which stay liquid. Common solid fats include butter, lard, tallow, suet, shortening, and, among plant fats, coconut oil and palm oil; the solid state comes from a high proportion of saturated fatty acids, whose straight molecular chains pack tightly together.
In cooking, solid fats have a higher smoke point than many liquid oils and are used for frying and searing. Their solid, workable texture is what gives baked goods like pie crust, biscuits, and cookies a flaky or tender crumb, since the fat can be cut into flour in pieces or creamed with sugar. They're also used to build emulsions such as mayonnaise and hollandaise.
Because of their saturated fat content, many recipes now offer liquid-oil substitutes such as olive or avocado oil, though the texture results won't be identical.