The X universe contains several races and factions - some are allies, some are enemies. In X4 some ships and weapons are restricted and require that you earn rank with the faction by trading, doing missions or assisting with killing criminals or enemies in their sectors.
As you increase your rank with a faction, you will receive a mission that tell you to visit the faction representative to receive a new rank. Simply land on the designated station, get up of your char (press ENTER and select GET UP) and then walk out on the station and down to the nearest teleporter (the orange mission beacon should guide you). Click the panel to the left and select the faction representative. Walk close to the representative, target him/her and click F and select menu choice 3 to accept your new rank.
Increasing your rank will unlock access to military and later capital ships at Wharfs and Shipyards controlled by the faction.
At any time you can visit the faction representatives and buy blueprints for building modules on your stations. If you forget where the different faction representatives are located, you can bookmark this page and check the table below.