The but-2-ene is a molecule with four carbons and the middle bond, (the second one) is a double-bond.
So, its structure is H₃C-CH=CH-CH₃.
However, the double bond makes two possible isomers, the "e" and "z":
The double bond, differently from the single bond, does not rotate, so these structures are not the same.
In the left one, the carbon groups have higher priority, and they are in alternating sides, bottom and top, and so it is called the E isomer.
In the right one, the carbon groups are on the same side, so it is called the Z isomer.