The racial guilds are, as their name says, guilds that are open only to the members of a certain race. The guilds teach basic skills and crafts that are considered important by the members of specific race.
Every race has its own racial guild and it must be completed before you can join any other guilds in your background. You can join your guild after completing the basic background training.
Note that you have to be level 11 to join your racial guild, and level 15 to max it (as it has five levels).
See list of race guilds.