torsional strain
Torsional strain is the small jolt of extra energy a molecule carries when bonds on neighboring atoms are forced to line up directly behind one another instead of nestling into the gaps. It is the discomfort of the eclipsed conformation — the reason a molecule would rather twist into a staggered shape, where its bonds avoid each other.
Why does eclipsing cost energy? Each covalent bond is a pair of electrons, and electron pairs repel. When a front bond eclipses a back bond, those two clouds of bonding electrons are pushed as close as rotation can put them, and they push back; the overlap of the bonds themselves is also unfavorable. The result is a modest energy penalty, around 4 kilojoules per mole for each pair of eclipsing carbon-hydrogen bonds in a simple molecule. Stack up three such pairs and you get ethane's roughly 12 kJ/mol eclipsed-to-staggered barrier.
Torsional strain is one of two strains conformational analysis weighs (the other being steric strain, from bulky groups crowding in space). Keep them distinct: torsional strain is about bonds eclipsing and appears even between tiny hydrogens, while steric strain needs genuinely large groups bumping. Torsional strain explains why ethane prefers staggered, contributes to the eclipsing peaks in butane and in rings, and is part of why the flat 'planar' drawing of cyclohexane is wrong — real cyclohexane puckers to let every bond stay staggered.
Eclipsed ethane carries about 12 kJ/mol of torsional strain from three eclipsing H-H pairs; rotate 60 degrees to the staggered form and that strain vanishes.
Torsional strain rises when bonds eclipse, and falls to near zero when they stagger.
Torsional strain (bonds eclipsing) is not the same as steric strain (bulky groups colliding) or angle strain (bond angles distorted). Naming the right strain is half of getting a conformation argument right.