A heker is something which makes something clear to people who see it. In regards to this halacha a heker is something on the table that reminds one not to eat the wrong food (שו”ע יו”ד פח:ב). If there is a heker on the table, it is permitted to eat meat at a table which also has milk. A heker is anything which either is not usually on the table, or something that is always on the table but not in this spot. For example, a vase that is usually in the middle of the table and was moved to the end would be a good heker. A heker does not work for a person eating by himself, only if there are two people eating together.
The heker does not have to be directly between them, it just has to be noticeable to those eating. The heker has to have some height. It cannot be something low such as a plate. In a case of need one can be lenient, as long as there is just a small amount of height. The exception to the rule of height is in place, if they are eating on separate placemats. A placemat is a good heker even though there is no height. A friend, who will watch you and remind you not to eat from the wrong food, does not help as a heker.