Printed material used to be created by using movable type. Each letter and character of a typeface was on a separate piece of wood or metal. The letter or character was placed on the base material as a mirror image of what it would look like when it was printed. To visualize raised, reversed type, think of a rubber stamp. The separate, movable letters and characters were arranged in lines of type by a typesetter. The size of the type was measured by measuring the piece of wood or metal on which it was placed from top to bottom.
That measurement was expressed in points. Ten-point type was smaller than 24-point type, and so on. Even though we no longer use this kind of type, we still use the measurement to express the size of type. For example, most paperbacks today are printed in 10-point to 12-point type. A one-inch high headline in a newspaper would be set in 72-point type, approximately.
An agate is a type size of 5-1/2 points. This is very small type. People with very good eyesight can read it with the naked eye. The rest of us need glasses or a magnifying glass. This size type is often used for classified ads in print newspapers.
A pica is a measurement of 12 points. Picas are used to measure the width and length of a page of type. A page in a trade paperback, for example, measures somewhere in the vicinity of 22 picas wide by 37 picas high.
Here's how these measurements relate to each other and to inches, the fourth scale on a printer's ruler. A pica is approximately 1/6 of an inch. There are about six picas in an inch. A point is 1/12 of a pica or approximately 1/72 of an inch. There are 12 points in a pica and 72 points in an inch. An agate is 5-1/2 points; there are approximately two agates in a pica and approximately 13 agates in an inch.
Read the point, agate and pica scales on a printer's ruler in the same way as you would read the inches scale, from left to right.