This achievement requires you to unlock each individual track for use in Free Race. To unlock a track, you have to complete a race on that track in Off-Road Masters.
The tracks in the game are as follows.
*** Spoiler - click to reveal ***Alaska
Through The Caves
Lumber Mill
Run To The Lake
Along The River
Pipeline - Layout 1
Fisherman's Village
Railroad - Layout 1
Pipeline - Layout 2
Railroad - Layout 2
Into The Woods
Namibia
Among The Dunes - Layout 1
Skeleton Coast - Layout 1
In The Savannah - Layout 1
In The Canyon
Among The Dunes - Layout 2
Crossing
Among The Dunes - Layout 3
Skeleton Coast - Layout 2
In The Savannah - Layout 2
In The Savannah - Layout 3
Blue Paradise: Pacific Ocean
The Oval
Long Layout
Figure 8
Short Layout
The Iron Mine: Outback
The Oval
Short Layout
Figure 8
Long Layout
Frozen Peak: Mont Blanc
The Oval
Short Layout
Figure 8
Long Layout
Los Angeles Memorial Coliseum
The Oval
Figure 8
The Crossing
The Double Oval
The Chicane
Florida Stadium
The Oval
The Spiral
Figure 8 - Layout 1
The Crossing
Figure 8 - Layout 2
Rallycross Loheac
Loheac
Autodromo Di Franciacorta
Franciacorta
Bikerniekv Trase Riga
Latvia
Las Vegas
Las Vegas
Laankebanen Helle
Hell
Grand Prix Trois-Revieres
Trois-Revieres
Rally San Luis
San Luis
Circuito Internacional De Montalegre
Montalegre
Estering Buxtehude
Estering
Unlocking tracks will come naturally as you progress through Off-Road Masters. However, you will not get around to unlocking the achievement until you get to the Final League episodes. If you're doing all the races in order, the last track you will unlock is
Mont Blanc - Long Layout in Episode 14 (Drivers On The Storm).