A lead jam is just that, lead jammed in the lead sleeve of a mechanical, or automatic, pencil, usually a drafting style automatic pencil as they have longer lead sleeves, generally 4mm's long. This longer lead sleeve can get jammed up, especially in the 0.3mm pencils, when the lead breaks inside the end cap, where the clutch is. Unknowingly the user keeps pressing the push button trying to advance more lead, which sometimes breaks and jams up the mouth of the lead sleeve. This jam can only be cleared with out damage to the pencil by using the clean out rod supplied with the pencil. The clean out rod resides under the eraser under the push button. It's a thin metal rod, about 3/4" to 7/8" long with a diameter the same, or smaller, as the lead in the pencil. The clean out rod for a 0.3mm pencil is 0.0145" in diameter and is often the size supplied with the larger diameter pencils as well and works just fine for all 4 lead sizes.
Some manufacturers have begun to omit the clean out rod from their pencils regardless of lead diameter so your pencil just may not have a clean out rod. If you want to make your own clean out rod you can obtain a 5-foot length of 0.0145" diameter stainless steel from either a local hardware store or on-line and cut them to about 7/8" long. Use a grinding stone or whetstone to de-burr and round off the cut end of the rod. This is most easily done by chucking the cut length in a pin vice. Leave just enough length exposed to work with so the rod will not bend under the pressure of being worked. Lightly drag the exposed end across the stone, twisting the pin vice as you do.
However the quickest and easiest way to obtain a pair of all-purpose clean out rods is to buy one of the following Pilot erasers sets: HERFS-10; HERF-10; or MS-10 from jetpens.com. These sets contain 5 erasers and 2 each clean-out-rods! They are about an inch long and terminate in a small loop at one end. Between the 2 sizes they will accommodate all 5 current thin-lead size pencils; 0.3-0.35 mm, 0.4 mm, 0.5 mm, 0.7 mm and 0.9 mm. Each set sells for under $2.00 and the erasers will fit a variety of different pencils. An eraser size chart is displayed when you bring up each of the 3 sets.
To clear a lead jam ready the clean out rod and then remove the pencil's end cap. Either place the base of the end cap on the desktop, or hold it between thumb and forefinger of your free hand with the base resting on the middle finger. Guide the clean out rod to the mouth of the lead sleeve then press it into the sleeve until the eraser contacts the lead sleeve to ensure that the jam is completely cleared. Remove the clean out rod and replace the end cap to resume the normal operation of the pencil.
Lead jams are infrequent, thankfully, so you may go a long time without ever needing to use a clean out rod, if ever. If your pencil has one then you can rest easy. However if it doesn't then the time to think about getting one is now, not when the need unexpectedly arises.