In the following the most stable conformation of n-butane is

* Fully eclipsed conformation is highly unstable due to,

1) Strong repulsions between front carbon C-H bonded electrons and back carbon C-H bonded electrons.These repulsions causes the strain in the molecule,which is said to be torsional strain or bond opposition strain.

2) And due to steric crowding between bulky methyl groups of front and back carbons.

* Anti conformation is highly stable,due to less torsional strain and less steric crowding between bulky methyl groups,since they are far away from each other.