Die Groot Trek (Afrikaans meaning The great trek in English) was an emigration wave from the British Cape colony northwest to the area of the Boer states. The representation in the game is quite ahistorical as the immigration was not a decision by the boer states but more an event that actually led to the foundation of those.
In the game the decision can be taken by either 
Oranje or 
Transvaal. If one takes it, the other has lost the chance to do so. It gives them 300 % immigrant attraction, +8 militancy for Boer POPs and gives Boer POPs in British cape colony with over 6 consciousness a modifier that makes them immigrate.
One needs at least 2 prestige to do so.
The decision paves the way to the 
Zulu decision Attack The Laagers, which reduces the amount of Boer POPs in Durban by 40%. Therefore, it is wise, if one intends to annex Zulu, to do so before enacting Die Groot Trek.