Chair is the most stable three dimensional conformation that a cyclohexane can take.
In the chair conformation, the bond angle for the carbons is about 109.5 degrees which is very close to the ideal bond angle for the sp3 hybridized carbons. Thus, a chair conformation minimizes angle strain. It also allows for the hydrogens to be in a staggered conformation which relieves torsional strain. There are actually two chair conformations that interchange rapidly in a process we call a chair flip or chair interconversion.
In their first semester of organic chemistry, students will learn how to correctly draw chairs, and how to convert 2 dimensional cyclohexane molecules with substituents into both chair forms.
The substituents on a chair can take axial and equatorial positions. Axial positions are vertical and either straight up or straight down. Equatorial positions are diagonal.
Substituents prefer to be on the equatorial positions because it relieves the 1,3 diaxial interactions (steric hindrance between hydrogens and substituents electron's cloud on the axial positions). The bigger the substituent is, the more it will want to be in the equatorial position.
How to figure out the most stable chair conformation?
1. Draw out the regular cyclohexane structure with its substituents. If the name says cis, put both substituents as either dash or wedge. If the name says trans, put one substituent as dash and another as wedge.
Dash will mean down and wedge will mean up when we draw chairs.
2. Draw a chair and number it, putting number 1 at the top right carbon and continue to number in the clockwise fashion. Looking at the cyclohexane structure you previously drew, add the substituents to the correct carbons in your chair. Remember that a dash means down and wedge means up. The substituents will fall either in axial or equatorial positions depending on whether they are up and down.
3. Next, draw a flipped chair. Start numbering from the carbon at the bottom right position and continue numbering clockwise. Again, looking at the cyclohexane structure you previously drew, add the substituents to the correct carbons in your chair.
4. Label the substituents on both chairs as either axial or equatorial. The chair with the most equatorial substituents wins and is the most stable.