Normal oral steroids are ineffective when administered orally bc of the hepatatic first pass metabolism of the liver. On the "first pass", these drugs are metabolized so much by the liver that by the time they reach the circulatory system, the concentration is much much lower and their basic structure has been broken down already. When you replace the hydrogen atom at the 17th alpha position with a carbon atom, this is referred to as alkylation which makes the steroid resistant to break down from the liver and therefore the first pass effect. Oftentimes this carbon atom is added with a methyl group (CH3) but you do see some ethyl groups added (C2H25).
Most of your common orals dbol, Winstrol (winny), Anavar, anadrol, etc require this to be effective. The alkylation process also changes the action of the compound. Some major changes you'll notice is an increase in tendency for it to produce estrogen related side effects even though it lowers the affinity for interaction with aromatase bc it makes a more potent form of estrogen. It lowers the propensity to bind to androgen receptors but by increasing the half life and therefore the the length of time it exists in an unbound state, it causes it to be a more potent anabolic and androgenic.
Non-methylated orals include Proviron, Primobolan, Andriole. Proviron and primo are alkylated at the 1 position with CH3 (methyl) which is a much weaker bond. The benefits of non-alkylated orals are obvious: very little relative liver toxicity but the cons are a much shorter half-life and they're less active in a mg for mg basis.